Tagged Pfizer Inc

Covid-19 Vaccine Side Effects: Your Questions Answered

The most common questions about vaccination side effects, answered.

Every day nearly three million people in the United States are getting the Covid-19 vaccine. And every new jab prompts new questions about what to expect after vaccination.

Last week I asked readers to send me their questions about vaccinations. Here are some answers.

Q: I’ve heard the Covid vaccine side effects, especially after the second dose, can be really bad. Should I be worried?

Short-lived side effects like fatigue, headache, muscle aches and fever are more common after the second dose of both the Pfizer-BioNTech and the Moderna vaccines, which each require two shots. (The Johnson & Johnson vaccine requires only a single shot.) Patients who experience unpleasant side effects after the second dose often describe feeling as if they have a bad flu and use phrases like “it flattened me” or “I was useless for two days.” During vaccine studies, patients were advised to schedule a few days off work after the second dose just in case they needed to spend a day or two in bed.

Data collected from v-safe, the app everyone is encouraged to use to track side effects after vaccination, also show an increase in reported side effects after the second dose. For instance, about 29 percent of people reported fatigue after the first Pfizer-BioNTech shot, but that jumped to 50 percent after the second dose. Muscle pain rose from 17 percent after the first shot to 42 percent after the second. While only about 7 percent of people got chills and fever after the first dose, that increased to about 26 percent after the second dose.

The New York Times interviewed several dozen of the newly vaccinated in the days afterward. They recounted a wide spectrum of responses, from no reaction at all to symptoms like uncontrolled shivering and “brain fog.” While these experiences aren’t pleasant, they are a sign that your own immune system is mounting a potent response to the vaccine.

Q: Is it true that women are more likely to get worse side effects from the vaccine than men?

An analysis of safety data from the first 13.7 million Covid-19 vaccine doses given to Americans found that side effects were more common in women. And while severe reactions to the Covid vaccine are rare, nearly all the cases of anaphylaxis, or life-threatening allergic reactions, occurred in women.

The finding that women are more likely to report and experience unpleasant side effects to the Covid vaccine is consistent with other vaccines as well. Women and girls can produce up to twice as many antibodies after receiving flu shots and vaccines for measles, mumps and rubella (M.M.R.) and hepatitis A and B. One study found that over nearly three decades, women accounted for 80 percent of all adult anaphylactic reactions to vaccines.

While it’s true that women may be more likely to report side effects than men, the higher rate of side effects in women also has a biological explanation. Estrogen can stimulate an immune response, whereas testosterone can blunt it. In addition, many immune-related genes are on the X chromosome, of which women have two copies and men have only one. These robust immune responses help to explain why 80 percent of autoimmune diseases afflict women. You can read more about women and vaccine side effects here.

Q: I didn’t have any side effects. Does that mean my immune system didn’t respond and the vaccine isn’t working?

Side effects get all the attention, but if you look at the data from vaccine clinical trials and the real world, you’ll see that many people don’t experience any side effects beyond a sore arm. In the Pfizer vaccine trials, about one out of four patients reported no side effects. In the Moderna trials, 57 percent of patients (64 or younger) reported side effects after the first dose — that jumped to 82 percent after the second dose, which means almost one in five patients reported no reaction after the second shot.

A lack of side effects does not mean the vaccine isn’t working, said Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the Food and Drug Administration’s vaccine advisory panel. Dr. Offit noted that during the vaccine trials, a significant number of people didn’t report side effects, and yet the trials showed that about 95 percent of people were protected. “That proves you don’t have to have side effects in order to be protected,” he said.

Nobody really knows why some people have a lot of side effects and others have none. We do know that younger people mount stronger immune responses to vaccines than older people, whose immune systems get weaker with age. Women typically have stronger immune responses than men. But again, these differences don’t mean that you aren’t protected if you don’t feel much after getting the shot.

Scientists still aren’t sure how effective the vaccines are in people whose immune systems may be weakened from certain medical conditions, such as cancer treatments or H.I.V. infection or because they are taking immune suppressing drugs. But most experts believe the vaccines still offer these patients some protection against Covid-19.

The bottom line is that even though individual immune responses can vary, the data collected so far show that all three vaccines approved in the United States — Pfizer-BioNTech, Moderna and Johnson & Johnson — are effective against severe illness and death from Covid-19.

Q: I took Tylenol before I had my Covid vaccine shots and had very little reaction to the shots. Did I make a big mistake?

You shouldn’t try to stave off discomfort by taking a pain reliever before getting the shot. The concern is that premedicating with a pain reliever like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin), which can prevent side effects like arm soreness as well as fever or headache, might also blunt your body’s immune response.

While it’s possible that taking a pain reliever before your shots might have dampened your body’s immune response, vaccine experts say you shouldn’t worry, and you shouldn’t try to get another shot. Studies of other vaccines suggest that while premedicating can dull the body’s immune response to a vaccine, your immune system can still mount a strong enough defense to fight infection. A review of studies of more than 5,000 children compared antibody levels in children who took pain relievers before and after vaccinations and those who did not. They found that pain relievers did not have a meaningful impact on immune response, and that children in both groups generated adequate levels of antibodies after their shots.

The high efficacy of all the Covid vaccines suggests that even if taking Tylenol before the shot did blunt your body’s immune response, there’s some wiggle room, and you are likely still well protected against Covid-19. “You should feel reassured that you’ll have enough of an immune response that you’ll will be protected, especially for vaccines that are this good,” said Dr. Offit.

Q: What about taking a pain reliever after the shot?

“It’s OK to treat” side effects with a pain reliever, said Dr. Offit, but if you don’t really need one, “don’t take it.”

While most experts agree it’s safe to take a pain reliever to relieve discomfort after you get vaccinated, they advise against taking it after the shot as a preventive or if your symptoms are manageable without it. The concern with taking an unnecessary pain reliever is that it could blunt some of the effects of the vaccine. (In terms of the vaccine, there’s no meaningful difference if you choose acetaminophen or ibuprofen.)

During the Moderna trial, about 26 percent of people took acetaminophen to relieve side effects, and the overall efficacy of the vaccine still was 94 percent.

Q: Are the side effects worse if you’ve already had Covid-19?

Research and anecdotal reports suggest that people with a previously diagnosed Covid-19 infection may have a stronger reaction and more side effects after their first dose of vaccine compared to those who were never infected with the virus. A strong reaction to your first dose of vaccine also might be a sign that you were previously infected, even if you weren’t aware of it.

If you previously tested positive for Covid-19 or had a positive antibody blood test, be prepared for a stronger reaction to your first dose, and consider scheduling a few days off work just in case. Not only will it be more comfortable to stay home and recover in bed, the vaccine side effects can resemble the symptoms Covid-19, and your co-workers won’t want to be near you anyway.

Q: I had Covid-19 already. Does that mean I can just get one dose?

Studies suggest that one dose might be adequate for people who have a previously confirmed case of Covid-19, but so far the medical guidelines haven’t changed. If you’ve received the Pfizer-BioNTech or Moderna vaccines, you should plan to get your second dose even if you’ve had Covid-19. Skipping your second dose could create problems if your employer or an airline ask to see proof of vaccination in the future. If you live in an area where the single-dose Johnson & Johnson vaccine is available, then you can be fully vaccinated after just one dose. You can read more here about the vaccine response in people who’ve had Covid-19.

Q: Will the vaccines work against the new variants that have emerged around the world?

The vaccines appear to be effective against a new variant that originated in Britain and is quickly becoming dominant in the United States. But some variants of the coronavirus, particularly one first identified in South Africa and one in Brazil, appear to be more adept at dodging antibodies in vaccinated people.

While that sounds worrisome, there’s reason to be hopeful. Vaccinated people exposed to a more resistant variant still appear to be protected against serious illness. And scientists have a clear enough understanding of the variants that they already are working on developing booster shots that will target the variants. The variants identified in South Africa and Brazil are not yet widespread in the United States.

People who are vaccinated should still wear masks in public and comply with public health guidelines, but you shouldn’t live in fear of variants, said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “If you’re vaccinated, you should feel pretty confident about how protected you are,” said Dr. Hotez. “It’s unlikely you’ll ever go to a hospital or an I.C.U. with Covid-19. In time you’re going to see a recommendation for a booster.”

I hope these answers will reassure you about your own vaccine experience. You can find a more complete list of questions and answers in our special vaccine tool “Answers to All Your Questions About Getting Vaccinated Against Covid-19.”

What to Expect When You Get Your Covid Vaccine

What to Expect When You Get Your Covid Vaccine

Kevin Mohatt for The New York Times

With more Americans becoming eligible for vaccination, many have questions about what the experience is like.

Here, Times science and health reporters answer some frequently asked questions about vaccination →

Mar. 10, 2021
Item 1 of 8
Swipe to continue reading →

The Pfizer-BioNTech Vaccine Is Said to Be Powerfully Protective in Adolescents

A clinical trial found no infections among vaccinated children ages 12 to 15, the companies said, and there were no serious side effects. The data have not yet been reviewed by independent experts.

The Pfizer-BioNTech coronavirus vaccine is extremely effective in adolescents 12 to 15 years old, perhaps even more so than in adults, the companies reported on Wednesday. No infections were found among children who received the vaccine in a recent clinical trial, the drug makers said; the children produced strong antibody responses and experienced no serious side effects.

The findings, if they hold up, may speed a return to normalcy for millions of American families. Depending on regulatory approval, vaccinations could begin before the start of the next academic year for middle school and high school students, and for elementary school children not long after.

The companies announced the results in a news release that did not include detailed data from the trial, which has not yet been peer-reviewed nor published in a scientific journal. Still, the news drew praise and excitement from experts.

“Oh my god, I’m so happy to see this — this is amazing,” said Akiko Iwasaki, an immunologist at Yale University. If the vaccines’s performance in adults was A-plus, the results in children were “A-plus-plus.”

The good news arrives even as the country records another rise in infections and health officials renew calls for Americans to heed precautions and get vaccinated. On Monday, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said that rising cases had left her with sense of “impending doom,” while President Biden called on state and local officials to reinstate mask mandates.

Vaccination efforts are accelerating throughout the nation. As of Tuesday, 29 percent of adults had received at least one dose of a coronavirus vaccine, and 16 percent had been fully inoculated, according to the C.D.C.

But the country cannot hope to reach herd immunity — the point at which immunity becomes so widespread that the coronavirus slows its crawl through the population — without also inoculating the youngest Americans, some experts say. Children under 18 account for about 23 percent of the population in the United States.

“The sooner that we can get vaccines into as many people as possible, regardless of their age, the sooner we will be able to really feel like we’re ending this pandemic for good,” said Angela Rasmussen, a virologist affiliated with Georgetown University in Washington.

Data from Israel suggest that vaccinating adults alone can significantly decrease the number of cases, but “long term, to hit the herd immunity threshold, we will have to vaccinate children,” she said.

The trial included 2,260 adolescents ages 12 to 15. The children received two doses of the vaccine three weeks apart — the same amounts and schedule used for adults — or a placebo of saltwater.

The researchers recorded 18 cases of coronavirus infection in the placebo group, and none among the children who received the vaccine. Still, the low number of infections makes it difficult to be too specific about the vaccine’s efficacy in the population at large, Dr. Rasmussen said.

“But obviously, it looks good for the vaccine if there were zero Covid cases among the vaccinated people,” she added.

The adolescents who got the vaccine produced much higher levels of antibodies on average, compared with participants 16 to 25 years of age in an earlier trial. The children experienced the same minor side effects as older participants, although the companies declined to be more specific.

Dr. Iwasaki said she had expected antibody levels in adolescents to be comparable to those in young adults. “But they’re getting even better levels from the vaccines,” she said. “That’s really incredible.”

She and other experts cautioned that the vaccine might be less effective in children, and adults, against some of the variants that have begun circulating in the United States.

Pfizer and BioNTech have begun a clinical trial of the vaccine in children under 12 and started inoculations of children ages 5 to 11 just last week. Company scientists plan to start testing the vaccine next week in even younger children, ages 2 to 5, followed by trials in children ages 6 months to 2 years.

Results from that three-phase trial are expected in the second half of the year, and the companies hope to make the vaccine available for children under 12 early next year.

“We share the urgency to expand the use of our vaccine to additional populations and are encouraged by the clinical trial data from adolescents between the ages of 12 and 15,” Albert Bourla, Pfizer’s chairman and chief executive officer, said in a statement.

Moderna has also been testing its vaccine in children. Results from a trial in adolescents ages 12 to 17 are expected in the next few weeks and in children 6 months to 12 years old in the second half of this year.

AstraZeneca started testing its vaccine in children 6 months and older last month, and Johnson & Johnson has said it will wait for results from trials in older children before testing its vaccine in children under 12.

Some parents have said they are reluctant to immunize their children because the risk posed by the virus is low. Children make up fewer than 1 percent of deaths from Covid-19, but about 2 percent of children who get the illness require hospital care.

The new results may not sway all of those parents, but they may reassure parents who have been wary of the vaccines, said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security.

“While I don’t think we have to wait until children are vaccinated to fully reopen schools, being able to vaccinate children may help some families feel safer about returning to school,” she said.

Pfizer and BioNTech plan to request from the Food and Drug Administration an amendment to the emergency use authorization for their vaccine, in hopes of beginning immunizations of older children before the start of the next school year. The companies also are planning to submit their data for peer review and publication in a scientific journal.

They will monitor the participants for two years after the second dose to assess the vaccine’s long-term safety and efficacy. Side effects of vaccines are usually apparent within the first six weeks, said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “Still, it’s good to know that safety monitoring is going to continue,” she said.

The C.D.C. recommends that people avoid getting other vaccines for two weeks before and after receiving the two doses of the coronavirus vaccine.

But children receive more vaccines in the few weeks before the school year than at any other time, Dr. Oliver noted, so pediatricians and parents should aim to get those other immunizations done earlier than usual.

The coronavirus vaccines should ideally be given by pediatricians who have deep experience in immunizing children, Dr. Oliver added. “Now is the time to start planning how that rollout is going to take place in this age group,” she said.

Getting One Vaccine Is Good. How About Mix-and-Match?

Researchers are exploring the possible benefits of pairing doses from two different Covid-19 vaccines.

In January, Britain made a change to its vaccine guidelines that shocked many health experts: If the second dose of one vaccine wasn’t available, patients could be given a different one.

The new rule was based on sheer guesswork; there was no scientific data at the time demonstrating that mixing two coronavirus vaccines was safe and effective. But that may change soon.

In February, researchers at the University of Oxford began a trial in which volunteers received a dose of the Pfizer-BioNTech vaccine followed by a dose of AstraZeneca’s formulation, or vice versa. This month, the researchers will start analyzing the blood of the subjects to see how well the mix-and-match approach works.

As growing numbers of vaccines are being authorized, researchers are testing other combinations. A few are in clinical trials, while others are being tested in animals for now.

Mixing vaccines might do more than just help overcome supply bottlenecks. Some researchers suspect that a pair of different vaccines might work better than two doses of the same one.

“I think we’re on the cusp of some interesting data,” said Adam Wheatley, an immunologist at the University of Melbourne in Australia.

The concept of mixing vaccines — sometimes called a heterologous prime-boost — is not new to our pandemic era. For decades, researchers have investigated the approach, hoping to find potent combinations against a range of viruses, such as influenza, H.I.V. and Ebola.

But scientists had little to show for all that research. It was easy enough to demonstrate that two vaccines may work well together in a mouse. But running full-blown clinical trials on a combination of vaccines is a tall order.

“For a single company to develop two parallel arms of a vaccine is twice the work and twice the cost,” Dr. Wheatley said.

Some of the early successes for heterologous prime-boosts came in the search for vaccines for Ebola. Many researchers focused their efforts on presenting the immune system with a protein found on the surface of the Ebola virus.

The gene for that protein was inserted into a different, harmless virus. When people received an injection of the vaccine, the harmless virus entered their cells; the cells then read the instructions in the Ebola gene and mass-produced Ebola’s surface protein. The immune system encountered the Ebola protein and made antibodies against it. And those antibodies protected the vaccinated people if they became infected with a full-blown Ebola virus.

This type of vaccine, called a viral vector vaccine, came with a big risk: The recipients might develop immunity to the viral vector after just the first dose. When the second dose arrived, their immune systems could swiftly wipe out the viral vector before it delivered its payload.

A number of vaccine makers decided to sidestep this potential threat by using different viruses for each dose. That way, the viral vectors in the second dose would be as new to the immune system as the first was. In 2017, for example, researchers at the Gamaleya Research Institute in Russia created an Ebola vaccine whose first dose contained a virus called an adenovirus. The second shot used another virus, called vesicular stomatitis virus.

When the Covid-19 pandemic began last year, the Gamaleya researchers used a similar strategy to create vaccines against the new coronavirus. The first dose used the same adenovirus as in their Ebola vaccine, called Ad5. The second dose contained a different human adenovirus, Ad26. The researches inserted a gene into both viruses for the protein on the surface of the coronavirus, called spike.

Studies revealed that the vaccine, now known as Sputnik V, provided a strong defense against Covid-19. In clinical trials, the researchers found that it had an efficacy of 91.6 percent. Sputnik V is now in use in Russia and 56 other countries.

Recently, the Gamaleya institute joined forces with AstraZeneca, which makes its own Covid-19 vaccine. AstraZeneca’s consists of two doses of a chimpanzee adenovirus called ChAdOx1. Last week, the company reported that its vaccine had an efficacy of 76 percent.

The Gamaleya and AstraZeneca teams want to see how well their vaccines work together. They have registered a pair of clinical trials in which volunteers will receive a dose of AstraZeneca’s ChAdOx1 and another of Sputnik V’s Ad26.

A spokesman for AstraZeneca said that one trial in Azerbaijan is underway, and a second, in Russia, is still under review by the country’s ministry of health.

Dr. Jakob Cramer, the head of clinical development at CEPI, a vaccine development organization, said that vaccines using viral vectors were not the only kind that might benefit from mixing. In fact, certain combinations might provoke a different, more effective immune response than a single type of vaccine. “Immunologically, there are several arguments in favor of exploring heterologous priming,” Dr. Kramer said.

Another kind of Covid-19 vaccine being tested contains the actual spike protein, rather than genetic instructions for it. Some of the vaccines contain the entire protein; others contain just a fragment of it. Currently, there are 29 protein-based vaccines for Covid-19 in clinical trials, although none have been authorized yet.

Dr. Wheatley and his colleagues have been testing protein-based vaccines in mice. They injected the full spike protein into the animals as a first dose. For the second dose, they injected only the tip of the spike, a region known as the receptor-binding domain, or R.B.D.

Dr. Wheatley and his colleagues found that the mixture worked better than two doses of the spike or of the R.B.D.

The researchers suspect that the first dose produces a broad range of antibodies that can stick to spots along the length of the spike protein, and that the second dose delivers a big supply of particularly potent antibodies to the tip of the spike. Together, the assortment of antibodies does a better job of stopping the coronavirus.

“You’re able to basically take that initial immunity that was elicited to that spike vaccine, and then really focus it down onto that R.B.D.,” Dr. Wheatley said.

Other combinations of vaccines may bring benefits of their own. Some vaccines, especially protein-based ones, do a good job of generating antibodies. Others, such as viral vectors, are better at training immune cells. A viral vector followed by a protein boost might offer the best of both worlds.

John Moore, a virologist at Weill Cornell Medicine, cautioned that there was no guarantee that clinical trials would reveal a benefit to mixing vaccines. In the search for an H.I.V. vaccine, researchers tried combining viral vectors and protein boost without success, he noted. Still, Dr. Moore said, the story might turn out differently for coronavirus vaccines.

“I’d like to see these studies done,” he said. “Doing it in the Covid space is completely rational, but may not be necessary.”

Some researchers are investigating heterologous vaccines not to find a superior mixture, but simply to open up more options for countries desperate to vaccinate their populations. Last week, India held back exports of vaccines to other countries as it grappled with a surge of Covid-19. For countries that were counting on those vaccines, a safe alternative for second doses could save lives.

After Britain was criticized in January for suggesting that vaccines could be mixed, researchers at the University of Oxford set out to put the idea to a formal test. In a trial called Com-Cov, they recruited 830 volunteers to test the two vaccines authorized by the British government: AstraZeneca’s adenovirus-based vaccine and the vaccine by Pfizer-BioNTech.

Pfizer-BioNTech’s vaccine uses a fundamentally different technology to produce spike proteins in the body. It contains tiny bubbles with genetic molecules called RNA. Once the bubbles fuse to cells, the cells use the RNA to make spike proteins.

One group of volunteers is receiving a Pfizer-BioNTech shot followed by AstraZeneca, while another will receive them in the reverse order. The other volunteers are receiving the standard two-dose version of the vaccines.

Later this month, the Oxford team will draw blood from the volunteers, examining their antibodies and immune cells to see whether the heterologous prime-boost creates an immune response roughly as strong as two doses of each of the authorized vaccines.

If more vaccines are authorized in Britain, the Com-Cov team may add them to the trial. Dr. Matthew Snape, who is leading the Com-COV trial, hopes it will be useful not just to his own country but to others that will be trying to vaccinate their citizens over the next few years.

“It might be that actually this flexibility becomes essential in the future,” he said.

Dr. Cramer said CEPI is planning to support additional heterologous prime-boost studies. There are plenty of possible studies to run. Worldwide, 13 vaccines are now in use against Covid-19, with 67 more in clinical trials.

“In the current situation, we have a quite a luxurious position of having so many advanced, effective vaccines,” Dr. Wheatley said.

As the number of authorized vaccines grows, the possible combinations in which they can be used will explode. Recently, researchers at China’s National Institutes for Food and Drug Control scaled up their research on heterologous prime-boosts by trying out four different vaccines that have either been authorized in China or are in late-stage clinical trials there — vaccines based on adenoviruses, proteins, RNA and coronaviruses that have been inactivated with chemicals.

The researchers injected mice with a first dose of one vaccine, then a second dose of another. Some of the combinations caused the mice to produce stronger immune responses than mice that received the same vaccine for both doses.

Whether scientists carry out more experiments on other vaccines will depend on the willingness of the vaccine manufacturers. “You’re requiring quite large pharmaceutical companies to play nice together,” Dr. Wheatley said.

Dr. Bernard Moss, a virologist at the National Institute of Allergy and Infectious Diseases, suspects that a number of companies will be willing to let their vaccines be tested in combinations. “It’s always better to be a part of something that is going to be used,” he said, “than to wholly own something that isn’t.”

One Vaccine Is Good. How About Mixing Two?

Researchers are exploring the possible benefits of combining doses from two different Covid-19 vaccines.

In January, Britain made a change to its vaccine guidelines that shocked many health experts: If the second dose of one vaccine wasn’t available, patients could be given a different one.

The new rule was based on sheer guesswork; there was no scientific data at the time demonstrating that mixing two coronavirus vaccines was safe and effective. But that may change soon.

In February, researchers at the University of Oxford began a trial in which volunteers received a dose of the Pfizer-BioNTech vaccine followed by a dose of AstraZeneca’s formulation, or vice versa. This month, the researchers will start analyzing the blood of the subjects to see how well the mix-and-match approach works.

As a growing number of vaccines are being authorized, researchers are testing other combinations. A few are in clinical trials, while others are being tested in animals for now.

Mixing vaccines might do more than just help overcome supply bottlenecks. Some researchers suspect that a pair of different vaccines might work better than two doses of the same one.

“I think we’re on the cusp of some interesting data,” said Adam Wheatley, an immunologist at the University of Melbourne in Australia.

The concept of mixing vaccines — sometimes called a heterologous prime-boost — is not new to our pandemic era. For decades, researchers have investigated the approach, hoping to find potent combinations against a range of viruses, such as influenza, H.I.V. and Ebola.

But scientists had little to show for all that research. It was easy enough to demonstrate that two vaccines may work well together in a mouse. But running full-blown clinical trials on a combination of vaccines is a tall order.

“For a single company to develop two parallel arms of a vaccine is twice the work and twice the cost,” Dr. Wheatley said.

Some of the early successes for heterologous prime-boosts came in the search for vaccines for Ebola. Many researchers focused their efforts on presenting the immune system with a protein found on the surface of the Ebola virus.

The gene for that protein was inserted into a different, harmless virus. When people received an injection of the vaccine, the harmless virus entered their cells; the cells then read the instructions in the Ebola gene and mass-produced Ebola’s surface protein. The immune system encountered the Ebola protein and made antibodies against it. And those antibodies protected the vaccinated people if they became infected with a full-blown Ebola virus.

This type of vaccine, called a viral vector vaccine, came with a big risk: The recipients might develop immunity to the viral vector after just the first dose. When the second dose arrived, their immune systems could swiftly wipe out the viral vector before it delivered its payload.

A number of vaccine makers decided to sidestep this potential threat by using different viruses for each dose. That way, the viral vectors in the second dose would be as new to the immune system as the first was. In 2017, for example, researchers at the Gamaleya Research Institute in Russia created an Ebola vaccine whose first dose contained a virus called an adenovirus. The second shot used another virus, called vesicular stomatitis virus.

When the Covid-19 pandemic began last year, the Gamaleya researchers used a similar strategy to create vaccines against the new coronavirus. The first dose used the same adenovirus as in their Ebola vaccine, called Ad5. The second dose contained a different human adenovirus, Ad26. The researches inserted a gene into both viruses for the protein on the surface of the coronavirus, called spike.

Studies revealed that the vaccine, now known as Sputnik V, provided a strong defense against Covid-19. In clinical trials, the researchers found that it had an efficacy of 91.6 percent. Sputnik V is now in use in Russia and 56 other countries.

Recently, the Gamaleya institute joined forces with AstraZeneca, which makes its own Covid-19 vaccine. AstraZeneca’s consists of two doses of a chimpanzee adenovirus called ChAdOx1. Last week, the company reported that its vaccine had an efficacy of 76 percent.

The Gamaleya and AstraZeneca teams want to see how well their vaccines work together. They have registered a pair of clinical trials in which volunteers will receive a dose of AstraZeneca’s ChAdOx1 and another of Sputnik V’s Ad26.

A spokesman for AstraZeneca said that one trial in Azerbaijan is underway, and a second, in Russia, is still under review by the country’s ministry of health.

Dr. Jakob Cramer, the head of clinical development at CEPI, a vaccine development organization, said that vaccines using viral vectors were not the only kind that might benefit from mixing. In fact, certain combinations might provoke a different, more effective immune response than a single type of vaccine. “Immunologically, there are several arguments in favor of exploring heterologous priming,” Dr. Kramer said.

Another kind of Covid-19 vaccine being tested contains the actual spike protein, rather than genetic instructions for it. Some of the vaccines contain the entire protein; others contain just a fragment of it. Currently, there are 29 protein-based vaccines for Covid-19 in clinical trials, although none have been authorized yet.

Dr. Wheatley and his colleagues have been testing protein-based vaccines in mice. They injected the full spike protein into the animals as a first dose. For the second dose, they injected only the tip of the spike, a region known as the receptor-binding domain, or R.B.D.

Dr. Wheatley and his colleagues found that the mixture worked better than two doses of the spike or of the R.B.D.

The researchers suspect that the first dose produces a broad range of antibodies that can stick to spots along the length of the spike protein, and that the second dose delivers a big supply of particularly potent antibodies to the tip of the spike. Together, the assortment of antibodies does a better job of stopping the coronavirus.

“You’re able to basically take that initial immunity that was elicited to that spike vaccine, and then really focus it down onto that R.B.D.,” Dr. Wheatley said.

Other combinations of vaccines may bring benefits of their own. Some vaccines, especially protein-based ones, do a good job of generating antibodies. Others, such as viral vectors, are better at training immune cells. A viral vector followed by a protein boost might offer the best of both worlds.

John Moore, a virologist at Weill Cornell Medicine, cautioned that there was no guarantee that clinical trials would reveal a benefit to mixing vaccines. In the search for an H.I.V. vaccine, researchers tried combining viral vectors and protein boost without success, he noted. Still, Dr. Moore said, the story might turn out differently for coronavirus vaccines.

“I’d like to see these studies done,” he said. “Doing it in the Covid space is completely rational, but may not be necessary.”

Some researchers are investigating heterologous vaccines not to find a superior mixture, but simply to open up more options for countries desperate to vaccinate their populations. Last week, India held back exports of vaccines to other countries as it grappled with a surge of Covid-19. For countries that were counting on those vaccines, a safe alternative for second doses could save lives.

After Britain was criticized in January for suggesting that vaccines could be mixed, researchers at the University of Oxford set out to put the idea to a formal test. In a trial called Com-Cov, they recruited 830 volunteers to test the two vaccines authorized by the British government: AstraZeneca’s adenovirus-based vaccine and the vaccine by Pfizer-BioNTech.

Pfizer-BioNTech’s vaccine uses a fundamentally different technology to produce spike proteins in the body. It contains tiny bubbles with genetic molecules called RNA. Once the bubbles fuse to cells, the cells use the RNA to make spike proteins.

One group of volunteers is receiving a Pfizer-BioNTech shot followed by AstraZeneca, while another will receive them in the reverse order. The other volunteers are receiving the standard two-dose version of the vaccines.

Later this month, the Oxford team will draw blood from the volunteers, examining their antibodies and immune cells to see whether the heterologous prime-boost creates an immune response roughly as strong as two doses of each of the authorized vaccines.

If more vaccines are authorized in Britain, the Com-Cov team may add them to the trial. Dr. Matthew Snape, who is leading the Com-COV trial, hopes it will be useful not just to his own country but to others that will be trying to vaccinate their citizens over the next few years.

“It might be that actually this flexibility becomes essential in the future,” he said.

Dr. Cramer said CEPI is planning to support additional heterologous prime-boost studies. There are plenty of possible studies to run. Worldwide, 13 vaccines are now in use against Covid-19, with 67 more in clinical trials.

“In the current situation, we have a quite a luxurious position of having so many advanced, effective vaccines,” Dr. Wheatley said.

As the number of authorized vaccines grows, the possible combinations in which they can be used will explode. Recently, researchers at China’s National Institutes for Food and Drug Control scaled up their research on heterologous prime-boosts by trying out four different vaccines that have either been authorized in China or are in late-stage clinical trials there — vaccines based on adenoviruses, proteins, RNA and coronaviruses that have been inactivated with chemicals.

The researchers injected mice with a first dose of one vaccine, then a second dose of another. Some of the combinations caused the mice to produce stronger immune responses than mice that received the same vaccine for both doses.

Whether scientists carry out more experiments on other vaccines will depend on the willingness of the vaccine manufacturers. “You’re requiring quite large pharmaceutical companies to play nice together,” Dr. Wheatley said.

Dr. Bernard Moss, a virologist at the National Institute of Allergy and Infectious Diseases, suspects that a number of companies will be willing to let their vaccines be tested in combinations. “It’s always better to be a part of something that is going to be used,” he said, “than to wholly own something that isn’t.”

Pfizer Begins Testing Its Vaccine in Young Children

Other drug makers have begun similar trials of their Covid-19 vaccines. If they work in children younger than 12 as expected, it will be easier for the U.S. to reach herd immunity.

Pfizer has begun testing its Covid-19 vaccine in children under 12, a significant step in turning back the pandemic. The trial’s first participants, a pair of 9-year-old twin girls, were immunized at Duke University in North Carolina on Wednesday.

Results from the trial are expected in the second half of the year, and the company hopes to vaccinate younger children early next year, said Sharon Castillo, a spokeswoman for the pharmaceutical company.

Moderna also is beginning a trial of its vaccine in children six months to 12 years of age. Both companies have been testing their vaccines in children 12 and older, and expect those results in the next few weeks.

AstraZeneca last month began testing its vaccine in children six months and older, and Johnson & Johnson has said it plans to extend trials of its vaccine to young children after assessing its performance in older children.

Immunizing children will help schools to reopen as well as help to end the pandemic, said Dr. Emily Erbelding, an infectious diseases physician at the National Institutes of Health who oversees testing of Covid-19 vaccines in special populations.

An estimated 80 percent of the population may need to be vaccinated for the United States to reach herd immunity, the threshold at which the coronavirus runs out of people to infect. Some adults may refuse to be vaccinated, and others may not produce a robust immune response.

Children under 18 account for about 23 percent of the population in the United States, so even if a vast majority of adults opt for vaccines, “herd immunity might be hard to achieve without children being vaccinated,” Dr. Erbelding said.

Pfizer had initially said it would wait for data from older children before starting trials of its vaccine in children under 12. But “we were encouraged by the data from the 12 to 15 group,” said Ms. Castillo, who did not elaborate on the results so far.

Scientists will test three doses of the Pfizer vaccine — 10, 20 and 30 micrograms — in 144 children. Each dose will be assessed first in children 5 through 11 years of age, then in children ages 2 through 4 years, and finally in the youngest group, six months to 2 years.

After determining the most effective dose, the company will test the vaccine in 4,500 children. About two-thirds of the participants will be randomly selected to receive two doses 21 days apart; the remaining will get two placebo shots of saline. The researchers will assess the children’s immune response in blood drawn seven days after the second dose.

“It sounds like a good plan, and it’s exciting that another Covid-19 vaccine is moving forward with trials in children,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York.

Dr. Oliver said about half of the parents she sees in practice are eagerly waiting for vaccines, and even to volunteer their children for clinical trials, while the rest are skeptical because comparatively few children become seriously ill from coronavirus infection.

Both groups of parents will benefit from knowing exactly how safe and effective the vaccines are in children, she said.

Children represent 13 percent of all reported cases in the United States. More than 3.3 million children have tested positive for the virus, at least 13,000 have been hospitalized and at least 260 have died, noted Dr. Yvonne Maldonado, who represents the American Academy of Pediatrics on the federal Advisory Committee on Immunization Practices.

The figures do not fully capture the damage to children’s health. “We don’t know what the long-term effects of Covid infection are going to be,” Dr. Maldonado said.

Other vaccines have helped to control many horrific childhood diseases that can cause long-term complications, she added: “For some of us who’ve seen that, we don’t want to go back to those days.”

Children often react more strongly to vaccines than adults do, and infants and toddlers in particular can experience high fevers. Any side effects are likely to appear soon after the shot, within the first week and certainly within the first few weeks, experts have said.

Some vaccines are tested only in animals before being assessed in children, and have to be monitored carefully for side effects.

“But this is a little different, because we’ve already had experience with tens of millions of people with these vaccines,” Dr. Maldonado said. “So there’s a higher degree of confidence now in giving this vaccine to kids.”

Some experts suggested that the Food and Drug Administration may require up to six months of safety data from studies of children before authorizing the Covid-19 vaccines. But a spokeswoman said the agency did not expect six months of safety data to support the vaccines’ authorization.

The Pfizer-BioNTech vaccine is authorized for children 16 through 18 years old, and the authorization for that age group was based on just two months of safety data, she said.

Parents will want to know how the companies and the F.D.A. plan to monitor and disclose side effects from the vaccines, and how long they will continue to follow trial participants after the vaccines’ authorization, Dr. Oliver said.

“I think everyone has learned throughout this,” she said. “The more transparent you can be, the better.”

Vaccinated People Can Get Covid, but It’s Most Likely Very Rare

“Breakthrough” cases, though quite uncommon, are a sharp reminder that vaccinated people should wear masks while the virus is circulating widely.

More than two months after he was fully vaccinated against Covid, a doctor in New York woke up with a headache and a dull, heavy feeling of fatigue. A fever and chills soon followed, and his senses of taste and smell began to fade.

This, he thought, could not be happening. But it was: He tested positive for the coronavirus.

“It was a huge shock,” he said. He knew that no vaccine was perfect and that the Pfizer-BioNTech shots he received had been found 95 percent effective in a large clinical trial. “But somehow in my mind, it was 100 percent,” he said.

The doctor, who requested anonymity to protect his privacy, is among the few reported cases of people who have been infected after being partly or even fully vaccinated. Nearly 83 million Americans have received at least one dose of a Covid vaccine, and it’s unclear just how many of them will have a “breakthrough” infection, though two new reports suggest the number is very small.

One study found that just four out of 8,121 fully vaccinated employees at the Southwestern Medical Center in Dallas became infected. The other found that only seven out of 14,990 workers at UC San Diego Health and the David Geffen School of Medicine at the University of California, Los Angeles tested positive two or more weeks after receiving a second dose of either the Pfizer-BioNTech or Moderna vaccines. Both reports, published on Tuesday in the New England Journal of Medicine, show how well the vaccines work in the real world, and during a period of intense transmission.

But these breakthrough cases, though quite rare, are a sharp reminder that vaccinated people are not invincible, especially when the virus continues to circulate widely.

“We felt really strongly that this data should not lead people to say, ‘Let’s all get vaccinated and then we can all stop wearing masks,’” said Dr. Francesca J. Torriani, an infectious disease specialist at UC San Diego Health who led the California study. “These measures have to continue until a larger segment of the population is vaccinated.”

Only some of the Covid-positive health workers in the California study showed symptoms, she said, and they tended to be mild, suggesting that the vaccines were protective. That echoes data from the vaccine trials indicating that breakthrough infections were mild and did not require hospitalizations. Some people had no symptoms at all, and were discovered only through testing in studies or as part of their medical care.

For example, doctors at the University of North Carolina found a few asymptomatic cases in vaccinated patients who were tested for the coronavirus ahead of surgery or other medical procedures, according to Dr. David Wohl, the medical director of that center’s vaccine clinic.

He said the absence of symptoms may have meant that the vaccine was doing exactly what it is supposed to do: prevent people from getting sick, even if it does not fully block the virus from infecting them.

The Centers for Disease Control and Prevention has a small team studying breakthrough cases, said an agency spokeswoman, Kristen Nordlund. One question the researchers are considering is whether particular variants of the coronavirus might play a role in breakthrough cases.

“Currently, there is no evidence that Covid-19 after vaccination is occurring because of changes in the virus,” Ms. Nordlund said.

In the next few months, Pfizer and Moderna are expected to release data that should indicate how often vaccinated people become infected by the virus, even if they have no symptoms. The companies have been testing participants in their vaccine trials for antibodies to a protein called N that is part of the coronavirus but not part of the vaccine. Finding those antibodies means that a vaccinated person has been infected by the virus. Some volunteers from the studies are also having their noses swabbed regularly to test for an active viral infection.

Another question is how effective the vaccines are in people whose immune systems have been weakened by illness or medications, said Dr. William Schaffner, an infectious disease expert at Vanderbilt University. Breakthrough cases might occur in those people because their bodies cannot produce a robust reaction to a vaccine.

“And it is amazing how pervasive immunocompromise is,” Dr. Schaffner said. He called the condition “a testament to modern medicine,” because many patients with it are being treated successfully for conditions that not so long ago would have killed them.

The doctor who became ill in New York despite full vaccination stayed in isolation at home for nearly two weeks. He described his illness as relatively mild, and said he was treated with monoclonal antibodies to fight the virus. “If the worst flu is a 10, this was a four,” he said.

Without the vaccine, he said, he believes he would have been sicker.

“I would have been in fear for my mortality,” he said. “But I didn’t have a moment’s anxiety. I did not think I was going to die. Thinking you’re not going to die — that’s a pretty big thing.”

Apoorva Mandavilli contributed reporting.

Some Long Covid-19 Patients Feel Better After Vaccine Doses

It is too soon to tell whether the shots have a broad beneficial effect on patients with continuing issues, but scientists are intrigued and beginning to study the phenomenon.

Judy Dodd began struggling with long Covid symptoms last spring — shortness of breath, headaches, exhaustion. Then she got the vaccine.

After her first Pfizer-BioNTech shot in late January, she felt so physically miserable that she had to be persuaded to get the second. For three days after that one, she also felt awful. But the fourth day, everything changed.

“I woke up and it was like ‘Oh what a beautiful morning,’” said Ms. Dodd, a middle-school teacher who is also an actor and director. “It was like I’d been directing ‘Sweeney Todd’ for months, and now I’m directing Oklahoma.”

Ms. Dodd, who continues to feel good, is among a number of people who are reporting that the post-Covid symptoms they’ve experienced for months have begun improving, sometimes significantly, after they got the vaccine. It’s a phenomenon that doctors and scientists are watching closely, but as with much about the yearlong coronavirus pandemic, there are many uncertainties.

Scientists are only beginning to study any potential effect of vaccines on long Covid symptoms. Anecdotes run the gamut: Besides those who report feeling better after the shots, many people say they have experienced no change and a small number say they feel worse.

Reports from doctors vary too. Dr. Daniel Griffin, an infectious disease physician at Columbia University, said about 40 percent of the long Covid patients he’s been treating cite symptom improvement after the vaccine. “They notice, ‘Hey, over the days, I’m feeling better. The fatigue isn’t so bad, maybe smell is coming back,’” Dr. Griffin said.

Other doctors say it is too early to know.

“Too few of our participants have been vaccinated so far to really be able to provide insight into this question,” said Dr. Michael Peluso, an infectious disease specialist working on a study of long-term Covid patients at University of California, San Francisco. “I’ve heard anecdotes as well, but I’ve seen too little data so far.”

This month, a small study by British researchers that has not yet been peer reviewed found that eight months after people were hospitalized for Covid-19, those who were vaccinated experienced improvement in more long Covid symptoms than those who weren’t yet vaccinated. The 44 vaccinated patients in the study were older and had more underlying medical conditions, since people with those characteristics qualified for vaccines earlier.

One month after vaccination, those patients reported improvement in 23 percent of their long Covid symptoms like joint pain and breathing, while 5.6 percent of their symptoms had worsened. The 22 unvaccinated people questioned at that time said 15 percent of their symptoms were better, while 14 percent of their symptoms were worse. There was no difference in response between people who received the Pfizer-BioNTech and Oxford-AstraZeneca vaccines.

Additional information comes from two surveys of several hundred people with long Covid symptoms, many of whom were never hospitalized for the disease.

Jim Golen, a former hospice nurse in Saginaw, Minn. He said his long Covid symptoms have gotten worse since his vaccination, but he’s still glad he got the vaccine.
Jim Golen, a former hospice nurse in Saginaw, Minn. He said his long Covid symptoms have gotten worse since his vaccination, but he’s still glad he got the vaccine.Tim Gruber for The New York Times

One survey of 345 people, mostly women and mostly in the U.K., found that two weeks or more after their second vaccine dose, 93 felt slightly better and 18 felt back to normal — a total of 32 percent reporting improved long Covid symptoms.

In that survey, by Gez Medinger, a London-based filmmaker who has experienced post-Covid symptoms, 61 people, just under 18 percent, felt worse, most of them reporting only a slight decline in their condition. Nearly half — 172 people — reported feeling no different.

Another survey, by Survivor Corps, a group of over 150,000 Covid survivors, found that as of March 17, 225 of 577 respondents reported some improvement, while 270 felt no change and 82 felt worse.

Jim Golen, 55, of Saginaw, Minn., feels some long Covid symptoms have worsened since his vaccination. Mr. Golen, a former hospice nurse who also has a small farm, had experienced months of difficulty, including blood clots in his lungs, chest pain, brain fog, insomnia and shortness of breath with any exertion. Late last year, after seeing several doctors, “I was finally starting to get better,” he said.

Since receiving the second dose of the Pfizer vaccine in mid-January, he said, his chest burning and shortness of breath have returned with a vengeance, especially if he taxes himself with activities like collecting sap from the maple trees on his farm. Nonetheless, Mr. Golen said he was “very happy” to be vaccinated, emphasizing that the effects of Covid were worse and preventing it is crucial.

Some people shared stories of stark symptom improvements that took them by surprise.

Laura Gross, 72, of Fort Lee, N.J. rattled off a lengthy list of debilitating long Covid symptoms she’d experienced since April, including exhaustion, joint pain, muscle aches and a “zizzy-dizzy-weaky thing that was like an internal headachy all-over-body vibration.”

Her cognitive fuzziness and forgetfulness were so intense that “brain fog barely describes it,” she said. “It’s more like brain cyclone.”

She also felt uncharacteristically “hopeless, sad, lonely, unmotivated,” she said.

Three days after her first Moderna shot in late January, everything changed. “It was like a revelation,” she said. The brain fog cleared completely, muscle aches were gone, joint pains were less intense and she suddenly had much more energy. It felt, she said, “like the old me.”

That continued after the second dose. “It’s like my cells went kerflooey last year when they met Covid,” she said, and the “vaccine said, ‘Wait, you dopes, that isn’t how you fight this, do it this way.’”

Recently, she walked briskly for 23 minutes and even “ran a little bit because I was so happy,” she said. “I’m a very happy little chappy.”

Laura Gross of Fort Lee, N.J., said brain fog is inadequate in describing her symptoms: “It’s more like brain cyclone,” she said. She reports feeling much better since being vaccinated.Nancy Borowick for The New York Times

Scientists say that understanding whether vaccines help some long Covid patients but not others could help unravel the underlying causes of different symptoms and potential ways to treat them.

“They might be different disease processes and you manage them differently,” said Dr. Adam Lauring, a virologist and infectious disease physician at the University of Michigan. “It might be that there’s a subset of people who have a certain type of long Covid, who respond well to vaccines, but there might be other people who have a different subtype that we haven’t quite defined yet.”

Akiko Iwasaki, an immunologist at Yale, said that a vaccine, by generating antibodies to the coronavirus’s spike protein, could potentially eliminate vestiges of the virus or remnants of viral RNA that may linger in some patients.

If this is occurring, she said, it could suggest that the vaccine “might be like a permanent cure” for those patients.

Dr. Iwasaki said the vaccine might also help people whose long Covid symptoms may be caused by a post-viral response resembling an autoimmune disease if “the vaccine stimulates innate immune responses that dampen these kinds of autoreactive responses,” she said. But based on experiences of people with other autoimmune diseases, that relief would “not be very long-lasting and they would kind of revert back” to having symptoms like fatigue, she said.

Dr. Eric Topol, a professor of molecular medicine at the Scripps Research Institute in San Diego, said he is starting a study to measure physiological information like heart rate, respiratory rate, temperature and markers of immune system response in people with long Covid before they receive a vaccine and weeks afterward.

It’s plausible that “you have your immune system revved up when you’re fighting a reservoir” of virus or RNA remnants, he said, “and that could be an explanation of why you’re in overdrive with your heart rate.” He wants to see if these biological indicators improve post-vaccine.

“We’d really like objective metrics that show that you not just feel better,” Dr. Topol said. “You could feel better from the placebo effect, but it’s unlikely your heart rate’s going to go from 100 to 60 because of a placebo effect. And if we keep seeing that pattern, that would be like Eureka.”

He added, “I think there’s probably something there, but I just don’t know what is the magnitude, how many people are going to benefit.”

There are many other questions: Are there specific characteristics, like age, gender, type or duration of symptoms, that might make some long Covid patients more likely to feel better? Would a vaccine be less effective for people with more complex conditions: people whose symptoms are driven by multiple biological pathways (perhaps both an RNA remnant and autoimmune activation) or whose symptoms have changed or fluctuated over time? Are certain types of vaccines more likely to produce benefit?

“It was awful thinking it may never get better, like ‘Is this my new normal, am I now damaged this way?’” recalled Bridget Hayward of her symptoms, which she says have eased since her vaccination.Alyssa Schukar for The New York Times

Bridget Hayward, 51, an operating room nurse in Alexandria, Va., said that after contracting Covid a year ago, her body ached from her hands to her hips and she became so brain-fogged that instead of asking for a scalpel, she would say “Give me that sharp thing we cut with.”

Almost daily, she would briefly pass out while bending down to fix a patient’s intravenous line or plug in the cord of a hospital bed.

“It was horrifying,” she said. “It was awful thinking it may never get better, like ‘Is this my new normal, am I now damaged this way?’”

After several months, her worst symptoms improved, but she still tired easily, felt hot even in cool weather, and found it too taxing to do some ordinary tasks, she said.

One day after her first dose of the Pfizer vaccine in late December, “it was like click, everything is fine,” she said. Her body temperature has normalized and “it felt like a darkness lifted.”

While “it’s not 100 percent every day,” she said she has so much energy now that “I’m not just getting from A to B, I’m like leaping up.”

One recent day, she did several long-overdue errands. “This may not sound like much but it is a 180-turnaround from three months ago,” she said. “I’m back!”

Kim Leighton, 64, of Vancouver, Wash., has had a similar experience. She was hospitalized last March and had long Covid symptoms that included mini blackouts, shortness of breath, getting lost in her own neighborhood, depression and fatigue.

“It really has been hell,” she said.

When she started feeling better in late January, she didn’t even think to connect it to the vaccine, but later realized her stark improvement had started four days after receiving her first Moderna shot. She is delighted that she can now take walks in downtown Portland and has the desire to reconnect with friends.

“Every day, I feel like I’m feeling stronger,” Ms. Leighton said. “All the stuff I had to let go of, I’m trying to get it back.”

Ms. Dodd, like several others, said she wasn’t taking her improvement for granted. “I’m still sort of wary of what’s around the corner, this disease is so unpredictable,” she said.

But, she added, “even if, God forbid, I have a relapse, to have this time now when I feel better, it’s really amazing.”

Some Long Covid Patients Feel Better After Getting the Vaccine

It is too soon to tell whether the shots have a broad beneficial effect on patients with continuing issues, but scientists are intrigued and beginning to study the phenomenon.

Judy Dodd began struggling with long Covid symptoms last spring — shortness of breath, headaches, exhaustion. Then she got the vaccine.

After her first Pfizer-BioNTech shot in late January, she felt so physically miserable that she had to be persuaded to get the second. For three days after that one, she also felt awful. But the fourth day, everything changed.

“I woke up and it was like ‘Oh what a beautiful morning,’” said Ms. Dodd, a middle-school teacher who is also an actor and director. “It was like I’d been directing ‘Sweeney Todd’ for months, and now I’m directing Oklahoma.”

Ms. Dodd, who continues to feel good, is among a number of people who are reporting that the post-Covid symptoms they’ve experienced for months have begun improving, sometimes significantly, after they got the vaccine. It’s a phenomenon that doctors and scientists are watching closely, but as with much about the yearlong coronavirus pandemic, there are many uncertainties.

Scientists are only beginning to study any potential effect of vaccines on long Covid symptoms. Anecdotes run the gamut: Besides those who report feeling better after the shots, many people say they have experienced no change and a small number say they feel worse.

Reports from doctors vary too. Dr. Daniel Griffin, an infectious disease physician at Columbia University, said about 40 percent of the long Covid patients he’s been treating cite symptom improvement after the vaccine. “They notice, ‘Hey, over the days, I’m feeling better. The fatigue isn’t so bad, maybe smell is coming back,’” Dr. Griffin said.

Other doctors say it is too early to know.

“Too few of our participants have been vaccinated so far to really be able to provide insight into this question,” said Dr. Michael Peluso, an infectious disease specialist working on a study of long-term Covid patients at University of California, San Francisco. “I’ve heard anecdotes as well, but I’ve seen too little data so far.”

This month, a small study by British researchers that has not yet been peer reviewed found that eight months after people were hospitalized for Covid-19, those who were vaccinated experienced improvement in more long Covid symptoms than those who weren’t yet vaccinated. The 44 vaccinated patients in the study were older and had more underlying medical conditions, since people with those characteristics qualified for vaccines earlier.

One month after vaccination, those patients reported improvement in 23 percent of their long Covid symptoms like joint pain and breathing, while 5.6 percent of their symptoms had worsened. The 22 unvaccinated people questioned at that time said 15 percent of their symptoms were better, while 14 percent of their symptoms were worse. There was no difference in response between people who received the Pfizer-BioNTech and Oxford-AstraZeneca vaccines.

Additional information comes from two surveys of several hundred people with long Covid symptoms, many of whom were never hospitalized for the disease.

Jim Golen, a former hospice nurse in Saginaw, Minn. He said his long Covid symptoms have gotten worse since his vaccination, but he’s still glad he got the vaccine.
Jim Golen, a former hospice nurse in Saginaw, Minn. He said his long Covid symptoms have gotten worse since his vaccination, but he’s still glad he got the vaccine.Tim Gruber for The New York Times

One survey of 345 people, mostly women and mostly in the U.K., found that two weeks or more after their second vaccine dose, 93 felt slightly better and 18 felt back to normal — a total of 32 percent reporting improved long Covid symptoms.

In that survey, by Gez Medinger, a London-based filmmaker who has experienced post-Covid symptoms, 61 people, just under 18 percent, felt worse, most of them reporting only a slight decline in their condition. Nearly half — 172 people — reported feeling no different.

Another survey, by Survivor Corps, a group of over 150,000 Covid survivors, found that as of March 16, 207 of 508 respondents reported some improvement, while 231 felt no change and 70 felt worse.

Jim Golen, 55, of Saginaw, Minn., feels some long Covid symptoms have worsened since his vaccination. Mr. Golen, a former hospice nurse who also has a small farm, had experienced months of difficulty, including blood clots in his lungs, chest pain, brain fog, insomnia and shortness of breath with any exertion. Late last year, after seeing several doctors, “I was finally starting to get better,” he said.

Since receiving the second dose of the Pfizer vaccine in mid-January, he said, his chest burning and shortness of breath have returned with a vengeance, especially if he taxes himself with activities like collecting sap from the maple trees on his farm. Nonetheless, Mr. Golen said he was “very happy” to be vaccinated, emphasizing that the effects of Covid were worse and preventing it is crucial.

Some people shared stories of stark symptom improvements that took them by surprise.

Laura Gross, 72, of Fort Lee, N.J. rattled off a lengthy list of debilitating long Covid symptoms she’d experienced since April, including exhaustion, joint pain, muscle aches and a “zizzy-dizzy-weaky thing that was like an internal headachy all-over-body vibration.”

Her cognitive fuzziness and forgetfulness were so intense that “brain fog barely describes it,” she said. “It’s more like brain cyclone.”

She also felt uncharacteristically “hopeless, sad, lonely, unmotivated,” she said.

Three days after her first Moderna shot in late January, everything changed. “It was like a revelation,” she said. The brain fog cleared completely, muscle aches were gone, joint pains were less intense and she suddenly had much more energy. It felt, she said, “like the old me.”

That continued after the second dose. “It’s like my cells went kerflooey last year when they met Covid,” she said, and the “vaccine said, ‘Wait, you dopes, that isn’t how you fight this, do it this way.’”

Recently, she walked briskly for 23 minutes and even “ran a little bit because I was so happy,” she said. “I’m a very happy little chappy.”

Laura Gross of Fort Lee, N.J., said brain fog is inadequate in describing her symptoms: “It’s more like brain cyclone,” she said. She reports feeling much better since being vaccinated.Nancy Borowick for The New York Times

Scientists say that understanding whether vaccines help some long Covid patients but not others could help unravel the underlying causes of different symptoms and potential ways to treat them.

“They might be different disease processes and you manage them differently,” said Dr. Adam Lauring, a virologist and infectious disease physician at the University of Michigan. “It might be that there’s a subset of people who have a certain type of long Covid, who respond well to vaccines, but there might be other people who have a different subtype that we haven’t quite defined yet.”

Akiko Iwasaki, an immunologist at Yale, said that a vaccine, by generating antibodies to the coronavirus’s spike protein, could potentially eliminate vestiges of the virus or remnants of viral RNA that may linger in some patients.

If this is occurring, she said, it could suggest that the vaccine “might be like a permanent cure” for those patients.

Dr. Iwasaki said the vaccine might also help people whose long Covid symptoms may be caused by a post-viral response resembling an autoimmune disease if “the vaccine stimulates innate immune responses that dampen these kinds of autoreactive responses,” she said. But based on experiences of people with other autoimmune diseases, that relief would “not be very long-lasting and they would kind of revert back” to having symptoms like fatigue, she said.

Dr. Eric Topol, a professor of molecular medicine at the Scripps Research Institute in San Diego, said he is starting a study to measure physiological information like heart rate, respiratory rate, temperature and markers of immune system response in people with long Covid before they receive a vaccine and weeks afterward.

It’s plausible that “you have your immune system revved up when you’re fighting a reservoir” of virus or RNA remnants, he said, “and that could be an explanation of why you’re in overdrive with your heart rate.” He wants to see if these biological indicators improve post-vaccine.

“We’d really like objective metrics that show that you not just feel better,” Dr. Topol said. “You could feel better from the placebo effect, but it’s unlikely your heart rate’s going to go from 100 to 60 because of a placebo effect. And if we keep seeing that pattern, that would be like Eureka.”

He added, “I think there’s probably something there, but I just don’t know what is the magnitude, how many people are going to benefit.”

There are many other questions: Are there specific characteristics, like age, gender, type or duration of symptoms, that might make some long Covid patients more likely to feel better? Would a vaccine be less effective for people with more complex conditions: people whose symptoms are driven by multiple biological pathways (perhaps both an RNA remnant and autoimmune activation) or whose symptoms have changed or fluctuated over time? Are certain types of vaccines more likely to produce benefit?

“It was awful thinking it may never get better, like ‘Is this my new normal, am I now damaged this way?’” recalled Bridget Hayward of her symptoms, which she says have eased since her vaccination.Alyssa Schukar for The New York Times

Bridget Hayward, 51, an operating room nurse in Alexandria, Va., said that after contracting Covid a year ago, her body ached from her hands to her hips and she became so brain-fogged that instead of asking for a scalpel, she would say “Give me that sharp thing we cut with.”

Almost daily, she would briefly pass out while bending down to fix a patient’s intravenous line or plug in the cord of a hospital bed.

“It was horrifying,” she said. “It was awful thinking it may never get better, like ‘Is this my new normal, am I now damaged this way?’”

After several months, her worst symptoms improved, but she still tired easily, felt hot even in cool weather, and found it too taxing to do some ordinary tasks, she said.

One day after her first dose of the Pfizer vaccine in late December, “it was like click, everything is fine,” she said. Her body temperature has normalized and “it felt like a darkness lifted.”

While “it’s not 100 percent every day,” she said she has so much energy now that “I’m not just getting from A to B, I’m like leaping up.”

One recent day, she did several long-overdue errands. “This may not sound like much but it is a 180-turnaround from three months ago,” she said. “I’m back!”

Kim Leighton, 64, of Vancouver, Wash., has had a similar experience. She was hospitalized last March and had long Covid symptoms that included mini blackouts, shortness of breath, getting lost in her own neighborhood, depression and fatigue.

“It really has been hell,” she said.

When she started feeling better in late January, she didn’t even think to connect it to the vaccine, but later realized her stark improvement had started four days after receiving her first Moderna shot. She is delighted that she can now take walks in downtown Portland and has the desire to reconnect with friends.

“Every day, I feel like I’m feeling stronger,” Ms. Leighton said. “All the stuff I had to let go of, I’m trying to get it back.”

Ms. Dodd, like several others, said she wasn’t taking her improvement for granted. “I’m still sort of wary of what’s around the corner, this disease is so unpredictable,” she said.

But, she added, “even if, God forbid, I have a relapse, to have this time now when I feel better, it’s really amazing.”

Plan to Ditch the Mask After Vaccination? Not So Fast.

Plan to Ditch the Mask After Vaccination? Not So Fast.

It’s not clear how easily vaccinated people may spread the virus, but the answer to that question is coming soon. Until then, scientists urge caution.

A health care worker prepared a dose of the Pfizer-BioNTech vaccine at a vaccination site in San Francisco on Monday.
A health care worker prepared a dose of the Pfizer-BioNTech vaccine at a vaccination site in San Francisco on Monday.Credit…Mike Kai Chen for The New York Times
Apoorva Mandavilli

  • March 3, 2021, 3:23 p.m. ET

With 50 million Americans immunized against the coronavirus, and millions more joining the ranks every day, the urgent question on many minds is: When can I throw away my mask?

It’s a deeper question than it seems — about a return to normalcy, about how soon vaccinated Americans can hug loved ones, get together with friends, and go to concerts, shopping malls and restaurants without feeling threatened by the coronavirus.

Certainly many state officials are ready. On Tuesday, Texas lifted its mask mandate, along with all restrictions on businesses, and Mississippi quickly followed suit. Governors in both states cited declining infection rates and rising numbers of citizens getting vaccinated.

But the pandemic is not yet over, and scientists are counseling patience.

It seems clear that small groups of vaccinated people can get together without much worry about infecting one another. The Centers for Disease Control and Prevention is expected shortly to issue new guidelines that will touch on small gatherings of vaccinated Americans.

But when vaccinated people can ditch the masks in public spaces will depend on how quickly the rates of disease drop and what percentage of people remain unvaccinated in the surrounding community.

Why? Scientists do not know whether vaccinated people spread the virus to those who are unvaccinated. While all of the Covid-19 vaccines are spectacularly good at shielding people from severe illness and death, the research is unclear on exactly how well they stop the virus from taking root in an immunized person’s nose and then spreading to others.

It’s not uncommon for a vaccine to forestall severe disease but not infection. Inoculations against the flu, rotavirus, polio and pertussis are all imperfect in this way.

The coronavirus vaccines “are under a lot more scrutiny than any of the previous vaccines ever have been,” said Neeltje van Doremalen, an expert in preclinical vaccine development at the National Institutes of Health’s Rocky Mountain Laboratories in Montana.

And now coronavirus variants that dodge the immune system are changing the calculus. Some vaccines are less effective at preventing infections with certain variants, and in theory could allow more virus to spread.

The research available so far on how well the vaccines prevent transmission is preliminary but promising. “We feel confident that there’s a reduction,” said Natalie Dean, a biostatistician at the University of Florida. “We don’t know the exact magnitude, but it’s not 100 percent.”

Still, even an 80 percent drop in transmissibility might be enough for immunized people to toss their masks, experts said — especially once a majority of the population is inoculated, and as rates of cases, hospitalizations and deaths plummet.

A line to register for a vaccination appointment in San Francisco. Experts say that people who have been inoculated should continue to wear masks to protect others.
A line to register for a vaccination appointment in San Francisco. Experts say that people who have been inoculated should continue to wear masks to protect others.Credit…Mike Kai Chen for The New York Times

But most Americans are still unvaccinated, and more than 1,500 people are dying every day. So given the uncertainty around transmission, even people who are immunized must continue to protect others by wearing masks, experts said.

“They should wear masks until we actually prove that vaccines prevent transmission,” said Dr. Anthony S. Fauci, director of the National Institute for Allergy and Infectious Diseases.

That proof is not yet in hand because the clinical trials for vaccines were designed to test whether the vaccines prevent serious illness and death, which usually reflects the virus’s impact on the lungs. Transmission, on the other hand, is driven by its growth in the nose and throat.

Primed by the vaccine, the body’s immune fighters should curb the virus soon after infection, shortening the infection period and curtailing the amounts in the nose and throat. That ought to significantly reduce the chances that a vaccinated person might infect others.

Animal studies support the theory. In one study, when monkeys were immunized and then exposed to the virus, seven of eight animals had no detectable virus in their noses or lung fluid, noted Juliet Morrison, a virologist at the University of California, Riverside.

Similarly, data from a few dozen participants in the Moderna trial who were tested when they got their second doses suggested that the first dose had decreased cases of infection by about two-thirds.

Another small batch of data emerged recently from the Johnson & Johnson trial. Researchers looked for signs of infection in 3,000 participants up to 71 days after getting the single-dose vaccine. Risk of infection in that study seemed to fall by about 74 percent.

“I think that’s very powerful,” said Dan Barouch, a virologist at Beth Israel Medical Center in Boston, who led one of the trial sites. “Those number estimates could change with more data, but the effect seems quite strong.”

More data is expected in the coming months from both Pfizer-BioNTech and Moderna.

But clinical trials may overestimate the power of a vaccine, because the type of people who choose to participate already tend to be careful and are counseled on precautions during the trial.

Some researchers instead are tracking infections among immunized people in real-world settings. For example, a study in Scotland conducted tests every two weeks, regardless of symptoms, on health care workers who had received the Pfizer-BioNTech vaccine. Investigators found that the vaccine’s effectiveness in preventing infection was 70 percent after one dose, and 85 percent after the second.

Researchers in Israel assessed infections in almost 600,000 immunized people and tried to trace their household contacts. The scientists found a 46 percent drop in infections after the first dose and a 92 percent drop after the second. (The study may have missed infections in people without symptoms.)

But to get a true assessment of transmission, researchers really need to know which immunized people become infected, and then trace the spread of the virus among their contacts with genetic analysis.

“That’s the ideal way to actually do this,” said Dr. Larry Corey, an expert in vaccine development at The Fred Hutchinson Cancer Research Center in Seattle. He is hoping to conduct such a study in college-age students.

Preparing swab samples at a testing site in San Francisco on Tuesday.Credit…Mike Kai Chen for The New York Times

But what precautions should immunized people take until the results from such studies become available? At the moment, many experts believe that what’s permissible will depend to a large extent on the number of cases in the surrounding community.

The higher the number of cases, the greater the likelihood of transmission — and the more effective vaccines must be in order to stop the spread.

“If the case numbers are zero, it doesn’t matter whether it’s 70 percent or 100 percent,” said Zoe McLaren, a health policy expert at the University of Maryland, referring to vaccine effectiveness.

Mask-wearing policies also will depend on how many unvaccinated people remain in the population. Americans may need to remain cautious as long as vaccination rates are low. But people will be able to relax a bit as those rates rise, and begin to return to normalcy once the virus runs out of others to infect.

“A lot of people have in mind that masks are the first thing that you let up on,” Dr. MacLaren said. In fact, she said, masks provide more freedom by allowing people to go to concerts, travel on buses or airplanes, or go shopping even with unvaccinated people around.

Ultimately, masks are a form of civic responsibility, said Sabra Klein, an immunologist at the Johns Hopkins Bloomberg School of Public Health.

“Are you wearing a mask to protect yourself from severe Covid, or are you wearing a mask for public health?” Dr. Klein said. “It’s right to do your part in the community beyond yourself.”

Why I Overcame My Vaccine Hesitancy

Personal Health

Why I Overcame My Vaccine Hesitancy

The more people who become immune to the virus, the less this scourge will be able to mutate and evade the vaccines already available

Credit…Gracia Lam
Jane E. Brody

  • March 1, 2021, 5:00 a.m. ET

Had I been polled last fall, I would have registered as a Covid vaccine skeptic. I told anyone who asked that I was going to wait at least six months after a vaccine was approved, by which time I hoped we’d know more about the degree and possibly the duration of its effectiveness and its potential side effects, especially in the elderly.

I was hardly alone in my concern that political influence might result in premature approval of a vaccine before its safety was well established.

Well, that hesitancy quickly dissipated after listening to reports from the directors of the Food and Drug Administration and the Centers for Disease Control and Prevention and several vaccine experts I know and trust, all of whom gave an enthusiastic thumbs-up to both the Pfizer and Moderna vaccines.

So in mid-January, when Gov. Andrew M. Cuomo of New York announced that residents 75 and older would qualify for the vaccine, I found a computerized link to schedule an appointment five days later.

At the mass vaccination site I went to in Brooklyn, everyone I encountered was cheerful, patient and reassuring, even the young woman checking me in who couldn’t find me on her list of 3 p.m. appointments. “Don’t worry,” she said reassuringly, “you’ll get the vaccine.”

At the next window, a young man from Nigeria checked my ID and Medicare card and figured out what had happened. Turned out I had inadvertently booked a 3 a.m. appointment, not realizing the site was open 24-7. Another “don’t worry,” and I moved on to a young technician from Florida who painlessly injected the Moderna vaccine into my left arm.

I then sat in a holding tent for 15 minutes to be sure I would have no serious reactions. The next day I got a text: “Hi Jane, It’s time for your daily v-safe check-in” and a link to a C.D.C. site that asked: How are you feeling today? (Good, Fair, Poor); Have you had a fever or felt feverish today? (yes, no); followed by a symptom check, first at the injection site for pain, redness, swelling or itching and then generally for chills, headache, joint pains, muscle or body aches, fatigue or tiredness, nausea, vomiting, diarrhea, abdominal pain and rash or any other symptoms I wanted to report.

Finally, I was asked several overall health impact questions about my ability to work and do my normal daily activities and whether I needed to consult a health care professional. I received the same text at the same time each day for more than a week, and was also given a link if I wanted to send a report to the Vaccine Adverse Event Reporting System.

The second dose, administered 34 days later, went even more smoothly. By then I’d spoken to dozens of others of various ages who had gotten both shots. Only two reported bad reactions — fever, nausea, extreme fatigue — that lasted a day or two. I was prepared for the worst, but it never happened. My arm, shoulder and neck hurt the first night, but most of the pain was gone the next morning. Although my son was on call in case I couldn’t walk my dog, his help wasn’t needed. I was even able to swim that afternoon.

But I assure you, even if I’d had a bad post-vaccine reaction, I would have sucked it up as a small price to pay for protection against a most devastating and too-often deadly disease like Covid-19. And I will continue to urge everyone and anyone I meet to do their damnedest to get immunized against Covid-19, especially now that potentially more potent variants are beginning to appear and spread.

The more people who become immune to the virus, the less this scourge will be able to mutate and evade the vaccines already available and the updated versions of vaccines scientists are now scrambling to produce.

Some people, hearing that vaccinated people may still be able to spread the infection and should continue to wear masks and practice social distancing which I will definitely do even after being fully immunized, question whether it pays to get the vaccine. Absolutely, it pays.

While there’s a chance that an immunized person might be able to infect others, existing evidence suggests the risk is very small. Far more important is unimpeachable data that the vaccines are lifesaving. They nearly eliminate the risk of severe illness, hospitalization and death from the virus. Of the 32,000 people who got the vaccine in the Pfizer and Moderna vaccine trials, only one person developed a severe case of Covid. Even if future mutations of the virus make an annual booster necessary, what’s the big deal? We already do that with the flu shot.

Keep in mind, too, that up to a third of people who develop Covid-19 can end up with debilitating symptoms that persist for many months, perhaps indefinitely for some. This is not always just a bug like the flu or common cold that is over and done with a week later. Eight months after recovering from a rather mild case of Covid, an otherwise healthy middle-aged friend said his lungs still hurt when he exerted himself.

Anecdotal reports that vaccine reactions are sometimes worse after the second of the two shots given for the Pfizer and Moderna vaccines have prompted some people to question whether they need both shots. Again, the only reliable evidence shows that two doses are necessary to achieve maximum protection. Do you buckle your seatbelt only every other time you’re in a car?

Now think of what will be possible once you, your family members and friends have had both shots and then waited the two weeks after the final dose to achieve the maximum level of immunity. While I’ll continue to wear a mask and socially distance in public, I’ll gladly dine indoors at home or in a well-spaced restaurant with fully immunized friends and relatives who are not considered at high risk.

I can’t wait to again attend live performances of classical music and opera and watch movies and plays not on my computer or TV but on the big screen and in the theater with others who can laugh, cry or sneer with me. But until most of us are reliably immune to the coronavirus that has terrified and secluded so many of us for so long, none of this can happen.

Meanwhile, I will continue to present the known facts and try to dispel misleading information about the vaccines. And I’ll hope that celebrities who enjoy the respect of vaccine-hesitant communities will be able to convince their members that controlling Covid infections and spread is critical, not just for their own sake, but for the future of their families, their towns, their country and a life that might again assume some semblance of normal.

Studies Suggest People Who Had Covid-19 Should Get Single Vaccine Dose

People Who Have Had Covid Should Get Single Vaccine Dose, Studies Suggest

New studies show that one shot of a vaccine can greatly amplify antibody levels in those who have recovered from the coronavirus.

A nurse supervisor prepared a dose of the Pfizer-BioNTech vaccine at a mass vaccination site in Hartford, Conn., this month.
A nurse supervisor prepared a dose of the Pfizer-BioNTech vaccine at a mass vaccination site in Hartford, Conn., this month.Credit…Christopher Capozziello for The New York Times
Apoorva Mandavilli

  • Feb. 19, 2021Updated 11:49 a.m. ET

Nearly 30 million people in the United States — and probably many others whose illnesses were never diagnosed — have been infected with the coronavirus so far. Should these people still be vaccinated?

Two new studies answer that question with an emphatic yes.

In fact, the research suggests that for these people just one dose of the vaccine is enough to turbocharge their antibodies and destroy the coronavirus — and even some more infectious variants.

The results of these new studies are consistent with the findings of two others published over the past few weeks. Taken together, the research suggests that people who have had Covid-19 should be immunized — but a single dose of the vaccine may be enough.

“I think it’s a really strong rationale for why people who were previously infected with Covid should be getting the vaccine,” said Jennifer Gommerman, an immunologist at the University of Toronto who was not involved in the new research.

A person’s immune response to a natural infection is highly variable. Most people make copious amounts of antibodies that persist for many months. But some people who had mild symptoms or no symptoms of Covid-19 produce few antibodies, which quickly fall to undetectable levels.

The vaccines “even the playing field,” Dr. Gommerman said, so that anyone who has recovered from Covid-19 produces enough antibodies to protect against the virus.

The latest study, which has not yet been published in a scientific journal, analyzed blood samples from people who have had Covid-19. The findings suggested that their immune systems would have trouble fending off B.1.351, the coronavirus variant first identified in South Africa.

But one shot of either the Pfizer-BioNTech or Moderna vaccine significantly changed the picture: It amplified the amount of antibodies in their blood by a thousandfold — “a massive, massive boost,” said Andrew T. McGuire, an immunologist at the Fred Hutchinson Cancer Research Center in Seattle, who led the study.

Flush with antibodies, samples from all of the participants could neutralize not only B.1.351, but also the coronavirus that caused the SARS epidemic in 2003.

In fact, the antibodies seemed to perform better than those in people who had not had Covid and had received two doses of a vaccine. Multiple studies have suggested that the Pfizer-BioNTech and Moderna vaccines are about five times less effective against the variant.

A Covid-19 patient in the intensive care unit of Marian Regional Medical Center in Santa Maria, Calif., this month.
A Covid-19 patient in the intensive care unit of Marian Regional Medical Center in Santa Maria, Calif., this month.Credit…Daniel Dreifuss for The New York Times

The researchers obtained blood samples from 10 volunteers in the Seattle Covid Cohort Study who were vaccinated months after contracting the coronavirus. Seven of the participants received the Pfizer-BioNTech vaccine and three received the Moderna vaccine.

Blood taken about two to three weeks after vaccination showed a significant jump in the amounts of antibodies compared with the samples collected before vaccination. The researchers don’t yet know how long the increased amount of antibodies will persist, but “hopefully, they’ll last a long time,” Dr. McGuire said.

The researchers also saw increases in immune cells that remember and fight the virus, Dr. McGuire said. “It looks pretty clear that we’re boosting their pre-existing immunity,” he said.

In another new study, researchers at New York University found that a second dose of the vaccine did not add much benefit at all for people who have had Covid-19 — a phenomenon that has also been observed with vaccines for other viruses.

In that study, most people had been infected with the coronavirus eight or nine months earlier, but saw their antibodies increase by a hundredfold to a thousandfold when given the first dose of a vaccine. After the second dose, however, the antibody levels did not increase any further.

“It’s a real testament to the strength of the immunologic memory that they get a single dose and have a huge increase,” said Dr. Mark J. Mulligan, director of the N.Y.U. Langone Vaccine Center and the study’s lead author.

In some parts of the world, including the United States, a significant minority of the population has already been infected, Dr. Mulligan noted. “They definitely should be vaccinated,” he said.

It’s unclear whether the thousandfold spike in antibody levels recorded in the lab will occur in real-life settings. Still, the research shows that a single shot is enough to increase the levels of antibodies significantly, said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.

Dr. Krammer led another of the new studies, which showed that people who have had Covid-19 and received one dose of a vaccine experienced more severe side effects from the inoculation and had more antibodies compared with those who had not been infected before.

“If you put all four papers together, that’s providing pretty good information about people who already had an infection only needing one vaccination,” Dr. Krammer said.

He and other researchers are trying to persuade scientists at the Centers for Disease Control and Prevention to recommend only one dose for those who have recovered from Covid-19.

A woman receives a vaccine at a drive-through site in Hartford.Credit…Christopher Capozziello for The New York Times

Ideally, those people should be monitored after the first shot in case their antibody levels plummet after some weeks or months, said Dennis R. Burton, an immunologist at the Scripps Research Institute in La Jolla, Calif.

The fact that the supercharged antibodies observed in the new study can fight the 2003 SARS virus suggests that a single dose of the vaccine may have prompted the volunteers’ bodies to produce “broadly neutralizing antibodies” — immune molecules capable of attacking a broad range of related viruses, Dr. Burton said.

He and other scientists have for decades investigated whether broadly neutralizing antibodies can tackle multiple versions of H.I.V. at once. H.I.V. mutates faster than any other virus and quickly evades most antibodies.

The new coronavirus mutates much more slowly, but there are now multiple variants of the virus that seem to have evolved to be more contagious or to thwart the immune system. The new study may provide clues on how to make a single vaccine that stimulates the production of broadly neutralizing antibodies that can destroy all variants of the coronavirus, Dr. Burton said.

Without such a vaccine, scientists will need to tweak the vaccines every time the virus changes significantly. “You’re stuck in a kind of Whac-a-Mole approach,” he said. It will probably take many months if not longer to develop and test that sort of vaccine against the coronavirus, but “that’s the longer-term way to approach this virus.”

People Who Have Had Covid Should Get Single Vaccine Dose, Studies Suggest

People Who Have Had Covid Should Get Single Vaccine Dose, Studies Suggest

New studies show that one shot of a vaccine can greatly amplify antibody levels in those who have recovered from the coronavirus.

A nurse supervisor prepared a dose of the Pfizer-BioNTech vaccine at a mass vaccination site in Hartford, Conn., this month.
A nurse supervisor prepared a dose of the Pfizer-BioNTech vaccine at a mass vaccination site in Hartford, Conn., this month.Credit…Christopher Capozziello for The New York Times
Apoorva Mandavilli

  • Feb. 19, 2021, 9:49 a.m. ET

At least 30 million people in the United States — and probably many others whose illnesses were never diagnosed — have been infected with the coronavirus so far. Should these people still be vaccinated?

Two new studies answer that question with an emphatic yes.

In fact, the research suggests that for these people just one dose of the vaccine is enough to turbocharge their antibodies and destroy the coronavirus — and even some more infectious variants.

The results of these new studies are consistent with the findings of two others published over the past few weeks. Taken together, the research suggests that people who have had Covid-19 should be immunized — but a single dose of the vaccine may be enough.

“I think it’s a really strong rationale for why people who were previously infected with Covid should be getting the vaccine,” said Jennifer Gommerman, an immunologist at the University of Toronto who was not involved in the new research.

A person’s immune response to a natural infection is highly variable. Most people make copious amounts of antibodies that persist for many months. But some people who had mild symptoms or no symptoms of Covid-19 produce few antibodies, which quickly fall to undetectable levels.

The vaccines “even the playing field,” Dr. Gommerman said, so that anyone who has recovered from Covid-19 produces enough antibodies to protect against the virus.

The latest study, which has not yet been published in a scientific journal, analyzed blood samples from people who have had Covid-19. The findings suggested that their immune systems would have trouble fending off B.1.351, the coronavirus variant first identified in South Africa.

But one shot of either the Pfizer-BioNTech or Moderna vaccine significantly changed the picture: It amplified the amount of antibodies in their blood by a thousandfold — “a massive, massive boost,” said Andrew T. McGuire, an immunologist at the Fred Hutchinson Cancer Research Center in Seattle, who led the study.

Flush with antibodies, samples from all of the participants could neutralize not only B.1.351, but also the coronavirus that caused the SARS epidemic in 2003.

In fact, the antibodies seemed to perform better than those in people who had not had Covid and had received two doses of a vaccine. Multiple studies have suggested that the Pfizer-BioNTech and Moderna vaccines are about five times less effective against the variant.

A Covid-19 patient in the intensive care unit of Marian Regional Medical Center in Santa Maria, Calif., this month.
A Covid-19 patient in the intensive care unit of Marian Regional Medical Center in Santa Maria, Calif., this month.Credit…Daniel Dreifuss for The New York Times

The researchers obtained blood samples from 10 volunteers in the Seattle Covid Cohort Study who were vaccinated months after contracting the coronavirus. Seven of the participants received the Pfizer-BioNTech vaccine and three received the Moderna vaccine.

Blood taken about two to three weeks after vaccination showed a significant jump in the amounts of antibodies compared with the samples collected before vaccination. The researchers don’t yet know how long the increased amount of antibodies will persist, but “hopefully, they’ll last a long time,” Dr. McGuire said.

The researchers also saw increases in immune cells that remember and fight the virus, Dr. McGuire said. “It looks pretty clear that we’re boosting their pre-existing immunity,” he said.

In another new study, researchers at New York University found that a second dose of the vaccine did not add much benefit at all for people who have had Covid-19 — a phenomenon that has also been observed with vaccines for other viruses.

In that study, most people had been infected with the coronavirus eight or nine months earlier, but saw their antibodies increase by a hundredfold to a thousandfold when given the first dose of a vaccine. After the second dose, however, the antibody levels did not increase any further.

“It’s a real testament to the strength of the immunologic memory that they get a single dose and have a huge increase,” said Dr. Mark J. Mulligan, director of the N.Y.U. Langone Vaccine Center and the study’s lead author.

In some parts of the world, including the United States, a significant minority of the population has already been infected, Dr. Mulligan noted. “They definitely should be vaccinated,” he said.

It’s unclear whether the thousandfold spike in antibody levels recorded in the lab will occur in real-life settings. Still, the research shows that a single shot is enough to increase the levels of antibodies significantly, said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.

Dr. Krammer led another of the new studies, which showed that people who have had Covid-19 and received one dose of a vaccine experienced more severe side effects from the inoculation and had more antibodies compared with those who had not been infected before.

“If you put all four papers together, that’s providing pretty good information about people who already had an infection only needing one vaccination,” Dr. Krammer said.

He and other researchers are trying to persuade scientists at the Centers for Disease Control and Prevention to recommend only one dose for those who have recovered from Covid-19.

A woman receives a vaccine at a drive-through site in Hartford.Credit…Christopher Capozziello for The New York Times

Ideally, those people should be monitored after the first shot in case their antibody levels plummet after some weeks or months, said Dennis R. Burton, an immunologist at the Scripps Research Institute in La Jolla, Calif.

The fact that the supercharged antibodies observed in the new study can fight the 2003 SARS virus suggests that a single dose of the vaccine may have prompted the volunteers’ bodies to produce “broadly neutralizing antibodies” — immune molecules capable of attacking a broad range of related viruses, Dr. Burton said.

He and other scientists have for decades investigated whether broadly neutralizing antibodies can tackle multiple versions of H.I.V. at once. H.I.V. mutates faster than any other virus and quickly evades most antibodies.

The new coronavirus mutates much more slowly, but there are now multiple variants of the virus that seem to have evolved to be more contagious or to thwart the immune system. The new study may provide clues on how to make a single vaccine that stimulates the production of broadly neutralizing antibodies that can destroy all variants of the coronavirus, Dr. Burton said.

Without such a vaccine, scientists will need to tweak the vaccines every time the virus changes significantly. “You’re stuck in a kind of Whac-a-Mole approach,” he said. It will probably take many months if not longer to develop and test that sort of vaccine against the coronavirus, but “that’s the longer-term way to approach this virus.”

Some Teens Volunteer for Covid Vaccine Trials to Get Their Lives Back

To Get Their Lives Back, Teens Volunteer for Vaccine Trials

Immunizing teenagers is a critical part of slowing the pandemic and reaching herd immunity. But enrolling them in clinical trials poses challenges that are very different than wrangling adults.

In Houston, Isabelle King, 14, gets her second dose of the Moderna vaccine from Jallesse Flores, as her twin sister,  Alexandra, looks on.
In Houston, Isabelle King, 14, gets her second dose of the Moderna vaccine from Jallesse Flores, as her twin sister,  Alexandra, looks on.Credit…Brandon Thibodeaux for The New York Times

  • Feb. 16, 2021, 1:55 p.m. ET

To get out of ninth-grade science period one recent Friday, the King twins had an excuse that is so very 2021.

Alexandra and Isabelle, 14, had to miss class — including a test — because they were participating in an actual science experiment: a clinical trial of Moderna’s Covid-19 vaccine to evaluate whether the shot is effective and safe in children ages 12 through 17.

“In science we’re learning about, like, genetics and stuff like that,” said Alexandra during the monitoring period after they’d gotten their shots at a Houston clinic. “So maybe the teacher will say, ‘Oh, you really shouldn’t have to take the test, because you’re contributing to science already.’”

Teenagers contract the novel coronavirus almost twice as often as younger children but vaccines authorized in the United States are mostly for adults — Moderna’s for 18 and older, Pfizer’s for 16 and up. While teenagers don’t become severely ill from the virus as often as adults, research suggests that because they are often asymptomatic and casual about social distancing, they can be efficient spreaders — to one another as well as to adults like parents, grandparents and teachers. Although vaccinating educators will be an important factor in keeping schools open, vaccinating students will also be a key element.

Bottom line: If widespread immunity to the coronavirus is to be achieved, adolescents are critical links. They need a Covid vaccine that works for them.

But teenagers are harder than adults to enroll and keep in clinical trials. They are difficult to wrangle and not so great with compliance, which includes keeping a symptom diary and keeping appointments, as many as six a year, that include blood draws (for some, an instant deal breaker).

To reach students, some researchers have tapped school connections, local pediatricians and social media campaigns. While waiting for appointments in the vaccine research clinics, some teenagers, ignoring advice to keep their vaccine volunteering off of social media, have posted TikTok videos, which have inspired friends to sign up. But the adolescent Covid vaccine trials will be much smaller than the adult trials — two or three thousand subjects instead of 30,000.

“You want to enroll as many kids as necessary but as few as possible, to expose as few as possible,” said Dr. Robert W. Frenck, director of the Vaccine Research Center at Cincinnati Children’s Hospital Medical Center.

To enroll in a trial, children must give “assent,” a legally required, age-appropriate version of “consent.” But researchers must also obtain the parent’s consent or permission. Information sessions for each can be protracted and painstaking. Objection by either child or parent terminates the application.

“Parents may be willing to put themselves in a study but not their children,” said Dr. James Campbell, a pediatric infectious disease expert at the University of Maryland School of Medicine who conducts vaccine trials. “Their key role in life, as parents, is to keep their children safe and they may choose to wait until a vaccine is licensed rather than enroll their child in a trial.”

Often researchers give young subjects an additional caution. Because vaccine disinformation and online personal attacks are so widespread, youth volunteers should avoid discussing their role in the trial on social media.

Audrey, 14, and Sam, 12, brother and sister, are participating in the Pfizer trial in Cincinnati.
Audrey, 14, and Sam, 12, brother and sister, are participating in the Pfizer trial in Cincinnati.Credit…Maddie McGarvey for The New York Times

Although the novel coronavirus has had far less impact on children than older adults, some 2.2 million pediatric cases in the United States have been reported and about 280 children have died, according to the Centers for Disease Control and Prevention. About 2,060 children have contracted a dangerous rare condition related to the coronavirus called multisystem inflammatory syndrome, which can shut down the heart and other organs.

And like anenemy occupation, the pandemic has taken over most children’s lives — shutting down in-person school, sports, socializing. That has prompted some teenagers, who otherwise feel so powerless, to fight back by volunteering for vaccine trials.

Sam, 12, who entered the Pfizer trial at Cincinnati Children’s hospital, said he wanted to participate “because it would be helping science and beat the pandemic. And it was my way of saying thank you to the frontline workers who are keeping us healthy.”

His sister, Audrey, 14, who is also in the study, said, “I thought this would be a really good story I could tell my children and grandchildren — that I tried to help create the vaccine.”

“And I also thought it is important to have people of different ages and races represented,” added Audrey, who, like her brother, is Asian. (Their mother, Rachel, a nurse researcher who volunteered for a vaccine trial, asked that their last names be withheld for privacy reasons.)

Overall, the teen trials may be less diverse, because results from adult trials showed no discernible difference in outcome by race. And because the adult trials were so successful, up to two-thirds of teenagers may be offered the actual vaccine rather than a placebo.

Pfizer, whose trial is fully enrolled, expects results from its trials for children ages 12 through 15 in the first quarter of this year, which it will then submit to the Food and Drug Administration for review. Moderna is still recruiting for its adolescent trials, with data anticipated sometime this summer. Other companies expect to start adolescent trials soon. Shortly after, researchers will open trials for children as young as 5, most likely with more modest doses.

As in any medical trial, investigators are evenhanded when discussing risks and benefits. Rather than lecturing young subjects, Dr. Campbell, whose clinic will conduct a Moderna trial for younger children, engages them in conversation.

“Do you remember your tetanus shot? Tell me about it,” he might say. And then, “Here’s how this is similar and how it’s different.” He wants to make sure the teenager is actively involved in decision-making. “We always say, ‘Don’t do this for your parents.’ ”

Dr. Sarah Hasan, lead recruiter for DM Clinical Research, which oversees the Houston Fights Covid campaign and most of the city’s vaccine trials, said that information sessions for adolescents and adults differ strikingly. She has more fun with the teenagers.

“Usually adults will skim the form, ask a few questions and they’re done,” she said. “But kids ask way more questions than adults and they’re actually listening, which is pretty nice.”

“Of course,” she added, “they also want to know if the doses will turn them into zombies.”

During adult trials, when participants were leery and results unknown, no-shows for appointments were common, Dr. Hasan said. But teenagers “come on time and are being super-compliant.”

Another issue in enrollment is compensation. Researchers typically offer volunteers money to cover time and travel expenses but the amount cannot be substantial enough to seem like an inducement. The going rate is about $15 an hour. The Cincinnati Pfizer trial compensates parents too, reasoning that they contribute time and money, as unofficial chauffeurs for the young subjects.

In Houston, the King sisters were already fantasizing about how they would spend the money.

Alexandra’s impulses were immediately generous. Now she could buy presents for others that her father typically paid for. “It’ll definitely be something nice to do, like, I earned it by getting a shot in the arm to help people. And now at least I can buy Christmas gifts for my mom,” she said.

Jonah Proctor, 15, awaits his shot at a Covid vaccine trial site in Houston.Credit…Brandon Thibodeaux for The New York Times

Her sister Isabelle hastily chimed in that, of course, she too would use the money for something selfless. Then she did allow, “Well, my closet could use an update.”

This summer, Monica Mitchell, a community health educator who works for Cincinnati Children’s, participated in a Pfizer vaccine trial. Her daughter Melanie, a high school junior, was deeply curious, but at that point, volunteers had to be at least 16 years old. On the day she turned 16, Melanie called to enroll.

Both mother and daughter have become ambassadors for the vaccine. “Someone said to me, ‘You’re doing the white man’s work,’ ” recounted Melanie, who is Black. “And I said, ‘No, it’s the opposite. I’m doing the work for my people.’ ”

Dr. Mitchell, who is also a pediatric psychologist and researcher, got some blowback for allowing her daughter to enter a vaccine trial. “Some relatives asked, “‘Why would you do that?’ But at least they understand that Melanie is fairly independent and they know she made her own decision,” Dr. Mitchell said.

Like most trial volunteers, children worry about side effects. Sure enough, after Sam got the second dose from Cincinnati Children’s hospital, he had a rough go of it.

In the middle of the night he woke with a throbbing headache. Then chills, a low-grade fever, muscle aches.

“He looked miserable,” said his mother, Rachel. “It’s one thing to talk theoretically about side effects but it was hard as a mom to see him feeling really bad.”

She felt guilty for having encouraged him to participate. “I’m so sorry,” she said.

Sam was mystified by her reaction. “I’m so happy,” he replied. “This means I got the real thing!”

Audrey, his sister, felt fine after her dose. “I’m jealous,” she said.

Covid Vaccines for Kids Are Coming, but Not for Many Months

Covid Vaccines for Kids Are Coming, but Not for Many Months

Pfizer and Moderna are testing their vaccines on children 12 and older and hope to have results by the summer.

A 15-year-old participating in Moderna’s teen Covid vaccine trial received a shot in Houston this month.
A 15-year-old participating in Moderna’s teen Covid vaccine trial received a shot in Houston this month.Credit…Brandon Thibodeaux for The New York Times
Apoorva Mandavilli

  • Feb. 12, 2021, 5:00 a.m. ET

As adults at high risk for Covid-19 line up to be immunized against the coronavirus, many parents want to know: When will my child get a vaccine?

The short answer: Not before late summer.

Pfizer and Moderna have enrolled children 12 and older in clinical trials of their vaccines and hope to have results by the summer. Depending on how the vaccines perform in that age group, the companies may then test them in younger children. The Food and Drug Administration usually takes a few weeks to review data from a clinical trial and authorize a vaccine.

Three other companies — Johnson & Johnson, Novavax and AstraZeneca — also plan to test their vaccines in children, but are further behind.

When researchers test drugs or vaccines in adults first, they typically then move down the age brackets, watching for any changes in the effective dose and for unexpected side effects.

“It would be pretty unusual to start going down into children at an early stage,” said Dr. Emily Erbelding, an infectious diseases physician at the National Institutes of Health who oversees testing of Covid-19 vaccines in special populations.

Some vaccines — those that protect against pneumococcal or meningococcal bacteria or rotavirus, for example — were tested in children first because they prevent pediatric diseases. But it made sense for coronavirus vaccines to be first tested in and authorized for adults because the risk of severe illness and death from Covid-19 increases sharply with age, said Paul Offit, a professor at the University of Pennsylvania and a member of the F.D.A.’s vaccine advisory panel.

“We’re trying to save lives, keep people out of the I.C.U., keep them from dying,” Dr. Offit said. That means prioritizing vaccines for the oldest people and for those with underlying conditions.

People younger than 21 account for about one-quarter of the population in the United States, but they make up less than 1 percent of deaths from Covid-19. Still, about 2 percent of children who get Covid-19 require hospital care, and at least 227 children in the United States have died of the disease.

“It is a significant disease in children, just not necessarily when you compare it to adults,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York.

Children will also need to be vaccinated in order for the United States to approach herd immunity — that long-promised goal at which the pandemic slows to a halt because the virus runs out of people to infect.

Scientists have estimated that 70 to 90 percent of the population might need to be immunized against the coronavirus to reach herd immunity, especially with more contagious variants expected to circulate widely in the country.

“Not all adults can get the vaccine because there’s some reluctance, or there’s maybe even some vulnerable immune systems that just don’t respond,” Dr. Erbelding said. “I think we have to include children if we’re going to get to herd immunity.”

It will also be important to immunize children in racial and ethnic populations that are hit hardest by the pandemic, she added.

Abhinav, 12, a participant in the Pfizer vaccine trial at Cincinnati Children’s Hospital last month.
Abhinav, 12, a participant in the Pfizer vaccine trial at Cincinnati Children’s Hospital last month.Credit…Cincinnati Children’s Hospital

Pfizer and Moderna’s clinical trials in adults each enrolled about 50,000 participants. They had to be that large in order to show significant differences between the volunteers who received a vaccine and those who got a placebo. But because it is rarer for children to become seriously ill with Covid-19, that kind of trial design in children would not be feasible, because it would require many more participants to show an effect.

Instead, the companies will look at vaccinated children for signs of a strong immune response that would protect them from the coronavirus.

The Pfizer-BioNTech vaccine was authorized in December for anyone 16 and older. The company has continued its trial with younger volunteers, recruiting 2,259 adolescents from 12 to 15 years of age. Teenagers are roughly twice as likely to be infected with the coronavirus as younger children, according to the Centers for Disease Control and Prevention.

Results from that trial should be available by summer, said Keanna Ghazvini, a spokeswoman for Pfizer.

“Moving below 12 years of age will require a new study and potentially a modified formulation or dosing schedule,” Ms. Ghazvini said. Those trials will most likely start later in the year, but the plans will be made final after the company has data from older children, she added.

Moderna’s vaccine, which was also authorized in December, is on a similar track for pediatric testing. In December, the company began testing adolescents ages 12 through 17, and plans to enroll 3,000 volunteers in this age group. The company expects results “around midyear 2021,” said Colleen Hussey, a spokeswoman for Moderna.

Based on the results, Moderna plans to assess the vaccine later this year in children between the ages of 6 months and 11 years.

Infants may have some antibodies at birth from vaccinated or infected mothers, but that maternal protection is unlikely to last through the first year of age. And with their relatively weak immune systems, babies might be particularly susceptible to infection if community transmission is high.

The trials will also assess the vaccine’s safety in children — and hopefully ease any fears that parents have. One-third of adults in the United States have said they do not plan to have their children immunized against the coronavirus, according to a recent poll conducted by Verywell Health.

Given the low risk of Covid-19 in children, some parents might be skeptical of the urgency to inoculate their children with a brand-new jab, Dr. Offit said. “For that reason, the vaccine would have to be held to a very high standard of safety,” he said.

More than 42 million people in the United States have been immunized so far, with few lasting side effects. And the F.D.A. has set up multiple systems to carefully monitor any serious reactions to the vaccine.

“They’re really looking at the data very, very closely,” Dr. Oliver said. “As a pediatrician and a mom, I have really good confidence that those systems work.”

Once a vaccine for children is available, schools can reintroduce extracurricular activities that involve close contact, like band practice, team sports and choir. But in the meantime, there is ample evidence that schools can reopen with other precautions in place, Dr. Oliver said.

“I don’t think we need to anticipate having a vaccine in order to open schools in the fall,” she said. “We should be planning now for opening schools.”

Dr. Oliver also urged parents to make sure children are immunized for other diseases. According to the C.D.C., orders for non-flu childhood vaccines through the Vaccines for Children Program are down approximately 10.3 million doses over all.

“Now’s the time to really catch up on missed doses of those vaccines,” she said. “Measles, HPV, tetanus boosters, pertussis boosters — all of that is really important.”

Which Covid Vaccine Should You Get? Experts Weigh the Effect Against Severe Disease

Which Covid Vaccine Should You Get? Experts Cite the Effect Against Severe Disease

Infectious disease doctors say getting a shot of the J&J vaccine, which has a lower efficacy against the virus than other vaccines, would still be well worthwhile.

Covid vaccinations in Santa Barbara County, Calif., this week.
Covid vaccinations in Santa Barbara County, Calif., this week.Credit…Daniel Dreifuss for The New York Times

  • Jan. 29, 2021, 5:32 p.m. ET

At first glance, the results reported on Friday from the long-awaited trial of Johnson & Johnson’s coronavirus vaccine might have seemed disappointing. Its overall efficacy — the ability to prevent moderate and severe disease — was reported at 72 percent in the United States, 66 percent in Latin American countries and 57 percent in South Africa.

Those figures appear far below the high bar set by Pfizer-BioNTech and Moderna, the first two vaccines authorized for emergency use in the United States, which reported overall efficacy from 94 to 95 percent.

Dr. Anthony S. Fauci, the nation’s leading infectious disease expert and now the lead medical adviser to President Biden on the coronavirus pandemic, acknowledged the striking difference at a briefing on Friday.

“If you woke up and you say, ‘Well, go to the door on the left and you get 94 or 95 percent, go to the door on the right and you get 72 percent,’ which door do you want to go to?” he asked.

But Dr. Fauci said that the more crucial measure was the ability to prevent severe disease, which translates to keeping people out of the hospital and preventing deaths. And that result, for Johnson & Johnson, was 85 percent in all of the countries where it was tested, including South Africa, where a rapidly spreading variant of the virus had shown some ability to elude vaccines.

More important than preventing “some aches and a sore throat,” Dr. Fauci said, is to fend off severe disease, especially in people with underlying conditions and in older adults, who are more likely to become seriously ill and to die from Covid-19.

“If you can prevent severe disease in a high percentage of individuals, that will alleviate so much of the stress in human suffering and death in this epidemic that we’re seeing, particularly now,” Dr. Fauci said, “as we well know, over the last several weeks, our health care system has been stressed by the number of people that require hospitalization, as well as intensive care.”

Dr. Francis Collins, director of the National Institutes of Health, compared the ability to prevent severe disease to the effects of flu shots, which do not always prevent influenza entirely but can make it less severe.

“The same thing seems to be applying here, in a circumstance where this variant is clearly making it a little tougher to get the most vigorous response that you would want to have,” Dr. Collins said. “But still, for severe disease, it’s looking really good.”

The Moderna vaccine also showed high efficacy, 100 percent, against severe disease. The Pfizer-BioNTech one appeared to as well, but the overall number of severe cases in the study was too small to be sure.

But researchers warn that trying to compare effectiveness between new studies and earlier ones may be misleading, because the virus is evolving quickly and to some extent the trials have studied different pathogens.

A participant in the Johnson & Johnson vaccine trial in Cape Town, South Africa, last month.
A participant in the Johnson & Johnson vaccine trial in Cape Town, South Africa, last month.Credit…Joao Silva/The New York Times

“You have to recognize that Pfizer and Moderna had an advantage,” Dr. William Schaffner, an infectious disease expert at Vanderbilt University, said in an interview. “They did their clinical trials before the variant strains became very apparent. Johnson & Johnson was testing its vaccine not only against the standard strain but they had the variants.”

The best way to stop the spread of mutants and to prevent more new ones from emerging is to vaccinate as many people as quickly as possible, Dr. Fauci and other researchers say. Viruses cannot mutate unless they are replicating, and they cannot replicate unless they can get into cells. Keeping them out by immunizing people can shut down the process.

In addition to the Pfizer-BioNTech and Moderna vaccines already in use in the United States, three more may soon become available: those made by Novavax, Johnson & Johnson and AstraZeneca. AstraZeneca’s vaccine has already been authorized in Britain and other countries.

Globally, the Johnson & Johnson vaccine is expected to play an important role, especially in low and middle-income countries, because it works after just one shot, is relatively inexpensive and is easier to store and distribute than the vaccines made by Pfizer-BioNTech and Moderna since it does not share their stringent requirements for freezing and refrigerating.

People waiting to be vaccinated may wonder if they will be able to pick and choose among vaccines, and if they should hold out and wait until the one that looks best to them becomes available.


Covid-19 Vaccines ›


Answers to Your Vaccine Questions

Currently more than 150 million people — almost half the population — are eligible to be vaccinated. But each state makes the final decision about who goes first. The nation’s 21 million health care workers and three million residents of long-term care facilities were the first to qualify. In mid-January, federal officials urged all states to open up eligibility to everyone 65 and older and to adults of any age with medical conditions that put them at high risk of becoming seriously ill or dying from Covid-19. Adults in the general population are at the back of the line. If federal and state health officials can clear up bottlenecks in vaccine distribution, everyone 16 and older will become eligible as early as this spring or early summer. The vaccine hasn’t been approved in children, although studies are underway. It may be months before a vaccine is available for anyone under the age of 16. Go to your state health website for up-to-date information on vaccination policies in your area

You should not have to pay anything out of pocket to get the vaccine, although you will be asked for insurance information. If you don’t have insurance, you should still be given the vaccine at no charge. Congress passed legislation this spring that bars insurers from applying any cost sharing, such as a co-payment or deductible. It layered on additional protections barring pharmacies, doctors and hospitals from billing patients, including those who are uninsured. Even so, health experts do worry that patients might stumble into loopholes that leave them vulnerable to surprise bills. This could happen to those who are charged a doctor visit fee along with their vaccine, or Americans who have certain types of health coverage that do not fall under the new rules. If you get your vaccine from a doctor’s office or urgent care clinic, talk to them about potential hidden charges. To be sure you won’t get a surprise bill, the best bet is to get your vaccine at a health department vaccination site or a local pharmacy once the shots become more widely available.

Probably not. The answer depends on a number of factors, including the supply in your area at the time you’re vaccinated. Check your state health department website for more information about the vaccines available in your state. The Pfizer and Moderna vaccines are the only two vaccines currently approved, although a third vaccine from Johnson & Johnson is on the way.

That is to be determined. It’s possible that Covid-19 vaccinations will become an annual event, just like the flu shot. Or it may be that the benefits of the vaccine last longer than a year. We have to wait to see how durable the protection from the vaccines is. To determine this, researchers are going to be tracking vaccinated people to look for “breakthrough cases” — those people who get sick with Covid-19 despite vaccination. That is a sign of weakening protection and will give researchers clues about how long the vaccine lasts. They will also be monitoring levels of antibodies and T cells in the blood of vaccinated people to determine whether and when a booster shot might be needed. It’s conceivable that people may need boosters every few months, once a year or only every few years. It’s just a matter of waiting for the data.

Employers do have the right to compel their workers to be vaccinated once a vaccine is formally approved. Many hospital systems, for example, require annual flu shots. But employees can seek exemptions based on medical reasons or religious beliefs. In such cases, employers are supposed to provide a “reasonable accommodation” — with a coronavirus vaccine, for example, a worker might be allowed to work if they wear a mask, or to work from home.

If you have other questions about the coronavirus vaccine, please read our full F.A.Q.

Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia, told CNN that if there was an abundant supply of the Pfizer-BioNTech and Moderna vaccines, they would be his first choices because of their higher overall efficacy.

But for now, there is not enough of those vaccines.

If he could not get either the Pfizer-BioNTech vaccine or the Moderna one, he would take the Johnson & Johnson shot, Dr. Offit said — as long as the data that the company will be presenting to the Food and Drug Administration looks as good as what the company reported on Friday.

He said Johnson & Johnson’s report of the reduction in severe disease was a powerful selling point.

“That’s what you want,” Dr. Offit said. “You want to stay out of the hospital, and stay out of the morgue.”

He noted that the company was also studying a two-shot regimen, which might raise its efficacy.

People who take the Johnson & Johnson vaccine should be able to safely receive a Pfizer-BioNTech or Moderna vaccine later if a booster shot is needed, he said.

Dr. Schaffner said he had just attended a meeting with other public health experts, and they had asked one another what they would tell their spouses or partners to do if they could get the Johnson & Johnson vaccine tomorrow, or had to wait three weeks for Pfizer-BioNTech’s or Moderna’s.

“All of us said, ‘Get the one tomorrow,’” Dr. Schaffner said. “The virus is bad. You’re risking three more weeks of exposure as opposed to getting protection tomorrow.”

He said Johnson & Johnson’s 85 percent efficacy against severe disease was a bit lower than those reported by Moderna and Pfizer-BioNTech, “but it’s still pretty darn high.”

It is not yet known whether it would be safe to take one type of vaccine now and then another later, Dr. Schaffner said, adding, “We haven’t studied this.”

Pregnant Women Get Conflicting Advice on Covid-19 Vaccines

The W.H.O. and the C.D.C. provide differing views, and experts partly blame a lack of data because expectant mothers have been excluded from clinical trials.

U.S. Vaccine Supply: What to Know

U.S. Covid Vaccine Supply: How to Make Sense of Those Confusing Numbers

Can President Biden really boost production? Why are governors trying to buy directly from Pfizer? And when will supply exceed demand?

New Yorkers lined up at a  vaccine hub set up by the city’s Health Department in Jamaica, Queens, this month.
New Yorkers lined up at a  vaccine hub set up by the city’s Health Department in Jamaica, Queens, this month.Credit…Hiroko Masuike/The New York Times

  • Jan. 21, 2021, 12:23 p.m. ET

Demand for vaccines is skyrocketing as the United States grapples with a record death toll from Covid-19 and the threat of new, more contagious variants. After a slow start in December, many states and cities have quickly ramped up vaccine delivery, widening access to larger groups of people and setting up mass testing sites.

But now there’s a new wrinkle: Some mayors and governors say they have run out of available vaccines, and have had to cancel appointments.

The Biden administration has promised to overhaul the country’s faltering vaccine effort, but there’s only so much it can do to increase the available supply.

Here’s what you need to know.

How many doses are available?

There are simply not enough doses of authorized vaccines to meet the enormous demand. And that is not likely to change for the next few months.

The two companies with authorized vaccines, Moderna and Pfizer, have each promised to provide the United States with 100 million vaccine doses by the end of March, or enough for 100 million people to get the necessary two shots.

But that doesn’t mean those 200 million doses are sitting in a factory warehouse somewhere, waiting to be shipped. Both companies are manufacturing the doses at full capacity, and are collectively releasing about 12 million doses each week, a number expected to gradually increase.

As of Wednesday, nearly 36 million doses of the Pfizer and Moderna vaccines had been distributed to state and local governments. However, only about 16.5 million shots had been administered to patients.

But as local health authorities become more adept at vaccine distribution, they will eventually catch up to the limited supply. Some local officials, including those in New York City, have said they are already reaching that point, and have had to cancel appointments because they said they do not have enough.

President Biden said he would use the Defense Production Act to increase supply. Will that help?

Vaccine experts and the companies themselves have said that at least in the short term, using the Defense Production Act will not significantly increase supply, although every little bit could help. That’s because manufacturing facilities are already at or near capacity, and there is a worldwide race to develop vaccines that use a finite amount of resources.

Although the Trump administration was criticized for not using the Defense Production Act more aggressively to ramp up production of testing supplies and protective gear, it did use the act many times to give vaccine manufacturers priority access to suppliers of raw ingredients and equipment.

In a plan released on Thursday, the Biden administration indicated it would continue to use the act to boost supplies needed for vaccine manufacturing, as well as other materials that are required to immunize tens of millions of people. Although the plan provided few details, one example cited is increasing production of a special syringe that can squeeze six doses out of Pfizer vials that were originally intended to contain five.

A drive-through vaccination center in Napa, Calif., earlier this month.
A drive-through vaccination center in Napa, Calif., earlier this month.Credit…Max Whittaker for The New York Times

What about the federal stockpile of vaccines?

There is no significant reserve of vaccines to speak of. For the most part, vaccines are being shipped out each week as they are manufactured. (The exception is a small emergency stockpile that the Biden administration has said will continue.)

Last week, Alex M. Azar II, the outgoing secretary of health and human services, stirred confusion when he announced that the federal government would be releasing a reserve of vaccine doses. Many states said they were told that this meant an influx of vaccines was on the way, which could be used to inoculate more people.

In his news conference, Mr. Azar urged states to open up their immunization policies, and said they had been moving too slowly to use the doses they had already been sent. As a result, several governors, including Andrew Cuomo in New York, changed eligibility rules to allow people 65 and older to get the vaccine.


Covid-19 Vaccines ›


Answers to Your Vaccine Questions

While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.

Yes, but not forever. The two vaccines that will potentially get authorized this month clearly protect people from getting sick with Covid-19. But the clinical trials that delivered these results were not designed to determine whether vaccinated people could still spread the coronavirus without developing symptoms. That remains a possibility. We know that people who are naturally infected by the coronavirus can spread it while they’re not experiencing any cough or other symptoms. Researchers will be intensely studying this question as the vaccines roll out. In the meantime, even vaccinated people will need to think of themselves as possible spreaders.

The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection won’t be any different from ones you’ve gotten before. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. But some of them have felt short-lived discomfort, including aches and flu-like symptoms that typically last a day. It’s possible that people may need to plan to take a day off work or school after the second shot. While these experiences aren’t pleasant, they are a good sign: they are the result of your own immune system encountering the vaccine and mounting a potent response that will provide long-lasting immunity.

No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

However, senior administration officials clarified last Friday that all of those reserve doses were already earmarked as booster shots for people who had gotten the vaccine, and that Mr. Azar was just spelling out the logical extension of a distribution policy that had been established by top federal officials in December, when shipments began. The release of the reserve doses would go to people who needed their second dose, not new pools of people who were getting their first shot.

Going forward, Mr. Azar said, the government would shift to a new model: rather than holding onto a reserve of booster shots, each weekly shipment from the manufacturers would include doses for new people as well as second doses for those due for their booster shots. President Biden echoed that policy in announcing his vaccine plan last week.

Will there be enough vaccine supply to give everyone a second dose?

Federal officials have previously said they were working with states to track who has gotten a vaccine, and when they are due for their booster shots, which is three weeks later for the Pfizer vaccine and four weeks later for the Moderna one.

They have said that each weekly shipment will give priority to people who need their second dose that week, and whatever is left will go to vaccinating new people.

But the plan relies on state and federal governments working together and accurately reporting who has received a vaccine, and what is needed from week to week. Many state governments have complained they do not have the resources to carry out the vaccine distribution plan, and the next few weeks will demonstrate how well the system works.

The incoming Biden administration has vowed to overhaul distribution to the states, providing more transparency to local officials about how much vaccine they can expect, in the hopes of allowing states to better plan.

Doses of Moderna’s vaccine were prepared at the Sonoma County Fairgrounds in Santa Rosa, Calif.Credit…Jim Wilson/The New York Times

Some Democratic governors have asked to buy vaccines directly from Pfizer. Is that possible?

No, it’s not likely to happen.

Last week, Gov. Gretchen Whitmer of Michigan asked the federal government for permission to buy 100,000 doses of vaccine directly from Pfizer. And on Monday, Gov. Cuomo wrote a letter to Pfizer asking for the state to buy vaccines directly.

Pfizer and Moderna’s supply has been fully claimed for at least the first quarter of this year, meaning it’s unlikely there will be any spare vaccine to sell to individual states.

In addition, the emergency use authorizations for the Pfizer and Moderna vaccines stipulate that the federal government oversees distribution.

In a statement, a Pfizer spokeswoman said the company “is open to collaborating with the U.S. Department of Health and Human Services on a distribution model that gives as many Americans as possible access to our vaccine as quickly as possible.” But she noted that “before we can even consider direct sales to state governments, H.H.S. would need to approve that proposal.”

A state official said on Tuesday that the governor felt it was important to exhaust all his options, no matter how unlikely they would be succeed, and pointed to his efforts in March to directly buy ventilators from manufacturers — setting up a bidding war among states that he later criticized the federal government for fueling.

But advisers to the Biden administration have indicated that they are not in favor of such a move. On Monday, Dr. Celine Gounder, a pandemic adviser to Mr. Biden during his presidential transition, said allowing states to reach separate deals would cause more problems than it would solve.

In an interview on CNBC, Dr. Gounder noted Mr. Cuomo’s previous criticism of bidding over ventilators. “I think this kind of an approach to vaccine allocation is going to result, frankly, in the same kind of situation that he, himself, was criticizing last spring,” she said.

Vaccinations underway in Minneapolis on Tuesday.Credit…Octavio Jones for The New York Times

Are we going to get more vaccines anytime soon?

Yes, most likely.

At least three other vaccines are in late-stage clinical trials, and the success of any one of them could mean millions of more doses for U.S. residents by this spring.

Johnson & Johnson is expected to announce the results of its vaccine trial any day now, and if it is successful, the first doses could become available in the United States by February. Although early production of the vaccine has lagged, the company has signed a deal to provide 100 million doses of its one-dose vaccine by the end of June.

By March and April, results from trials testing two-dose vaccines by AstraZeneca and Novavax could also be made public. AstraZeneca has an arrangement with the U.S. government to provide 300 million doses, and Novavax to provide 110 million.

What’s more, both Pfizer and Moderna say their factories are ramping up and expanding capacity each week. They have signed deals to supply an additional 100 million doses each of their vaccines in the second quarter of this year.

When will we have enough vaccines for everybody in the country?

It’s still not clear, although conservatively, there could be enough vaccines by the summer.

If no other vaccines are authorized, the United States has signed deals with Pfizer and Moderna for a total of 400 million doses to be delivered by summer, or enough for 200 million people.

That’s pretty close to the American population of 260 million adults (the vaccines are not approved yet for children although studies are underway).

But if other vaccines do prove safe and effective — which experts say is likely — millions more people could be vaccinated more quickly, possibly by late spring.

Emerging Coronavirus Variants May Pose Challenges to Vaccines

Emerging Coronavirus Variants May Pose Challenges to Vaccines

Laboratory studies of mutations circulating in South Africa suggest they may dodge some of the body’s immune responses.

Health workers tended to a Covid-19 patient at Greenacres Hospital in Port Elizabeth, South Africa, in November.
Health workers tended to a Covid-19 patient at Greenacres Hospital in Port Elizabeth, South Africa, in November.Credit…Samantha Reinders for The New York Times
Apoorva Mandavilli

  • Jan. 20, 2021, 6:37 p.m. ET

The steady drumbeat of reports about new variants of the coronavirus — first in Britain, then in South Africa, Brazil and the United States — have brought a new worry: Will vaccines protect against these altered versions of the virus?

The answer so far is yes, several experts said in interviews. But two small new studies, posted online Tuesday night, suggest that some variants may pose unexpected challenges to the immune system, even in those who have been vaccinated — a development that most scientists had not anticipated seeing for months, even years.

The findings result from laboratory experiments with blood samples from groups of patients, not observations of the virus spreading in the real world. The studies have not yet been peer-reviewed.

But experts who reviewed the papers agreed that the findings raised two disturbing possibilities. People who had survived mild infections with the coronavirus may still be vulnerable to infection with a new variant; and more worryingly, the vaccines may be less effective against the variants.

Existing vaccines will still prevent serious illness, and people should continue getting them, said Dr. Michel Nussenzweig, an immunologist at Rockefeller University in New York, who led one of the studies: “If your goal is to keep people out of the hospital, then this is going to work just fine.”

But the vaccines may not prevent people from becoming mildly or asymptomatically infected with the variants, he said. “They may not even know that they were infected,” Dr. Nussenzweig added. If the infected can still transmit the virus to others who are not immunized, it will continue to claim lives.

The vaccines work by stimulating the body to produce antibodies against the coronavirus. Scientists had expected that over time, the virus may gain mutations that allow it to evade these antibodies — so-called escape mutations. Some studies had even predicted which mutations would be most advantageous to the virus.

But scientists had hoped that the new vaccines would remain effective for years, on the theory that the coronavirus would be slow to develop new defenses against them. Now some researchers fear the unchecked spread has given the virus nearly unfettered opportunities to reinvent itself, and may have hastened the appearance of escape mutations.

The studies published on Tuesday night show that the variant identified in South Africa is less susceptible to the antibodies created by natural infection and by vaccines made by Pfizer-BioNTech and Moderna.

Vaccinations of nurses, doctors and health professionals in São Paulo, Brazil, this week.
Vaccinations of nurses, doctors and health professionals in São Paulo, Brazil, this week.Credit…Victor Moriyama for The New York Times

Neither the South African variant nor a similar mutant virus in Brazil has yet been detected in the United States. (The more contagious variant that has blazed through Britain does not contain these mutations and seems to be susceptible to vaccines.)

Fears that the vaccines would be powerless against new variants intensified at a scientific conference held online on Saturday, when South African scientists reported that in laboratory tests, serum samples from 21 of a group of 44 Covid-19 survivors did not destroy the variant circulating in that country.

The samples that were successful against the variant were taken from patients who had been hospitalized. These patients had higher blood levels of so-called neutralizing antibodies — the subset of antibodies needed to disarm the virus and prevent infection — than those who were only mildly ill.

The results “strongly, strongly suggest that several mutations that we see in the South Africa variant are going to have a significant effect on the sensitivity of that virus to neutralization,” said Penny Moore, a virologist at the National Institute for Communicable Diseases in South Africa who led the study.

The second study brought better tidings, at least about vaccines.

In that study, Dr. Nussenzweig and his colleagues tested samples from 14 people who had received the Moderna vaccine and six people who had received the Pfizer-BioNTech vaccine.


Covid-19 Vaccines ›


Answers to Your Vaccine Questions

While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.

Yes, but not forever. The two vaccines that will potentially get authorized this month clearly protect people from getting sick with Covid-19. But the clinical trials that delivered these results were not designed to determine whether vaccinated people could still spread the coronavirus without developing symptoms. That remains a possibility. We know that people who are naturally infected by the coronavirus can spread it while they’re not experiencing any cough or other symptoms. Researchers will be intensely studying this question as the vaccines roll out. In the meantime, even vaccinated people will need to think of themselves as possible spreaders.

The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection won’t be any different from ones you’ve gotten before. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. But some of them have felt short-lived discomfort, including aches and flu-like symptoms that typically last a day. It’s possible that people may need to plan to take a day off work or school after the second shot. While these experiences aren’t pleasant, they are a good sign: they are the result of your own immune system encountering the vaccine and mounting a potent response that will provide long-lasting immunity.

No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

The researchers saw a slight decrease in antibody activity directed against engineered viruses with three of the key mutations in the variant identified in South Africa. That result was significant “because it’s seen in just about every individual tested,” Dr. Nussenzweig said. Still, it “is not something that we should be horribly freaked out about.”

In most people, infection with the coronavirus leads to a strong immune response; the vaccines seem to induce an even more powerful response. Two doses of the vaccines from Pfizer and Moderna, at least, produce neutralizing antibodies at levels that are higher than those acquired through natural infection.

Even if antibody effectiveness were reduced tenfold, the vaccines would still be quite effective against the virus, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

In Liverpool, England, people lined up to receive a coronavirus test at a recreational tennis center last year.Credit…Mary Turner for The New York Times

And while neutralizing antibodies are essential for preventing infection, the vaccines — and natural infection — also lead to production of thousands of other types of antibodies, not to mention various immune cells that retain a memory of the virus and can be roused to action when the body encounters it again.

Even when confronted with variants, those other components of the immune system may be enough to prevent serious illness, said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York. In clinical trials, the vaccines protected people from illness after just one dose, when the levels of neutralizing antibodies were low or undetectable, he noted.

Vaccine trials being conducted in South Africa by Novavax and Johnson & Johnson will provide more real-world data on how the vaccines perform against the new variant there. Those results are expected within the next few weeks.

All viruses mutate, and it’s no surprise that some of those mutations sidestep the body’s immune defenses, experts said. Each new host affords a virus fresh opportunities to amass and test mutations by slightly scrambling the sequence of RNA letters in its genetic code.

“The beauty, the elegance, the evolution and the magnificence of a virus is that every single time it infects a person, it’s exploring that sequence space,” said Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh.

Some mutations don’t improve on the original, and fade away. Others add to the pathogen’s power, by making it more contagious — like the variant first identified in Britain — more fit, or less susceptible to immunity.

The mutations in the variant circulating in South Africa, called B.1.351, have independently emerged more than once, and all together, suggesting that they work in concert to benefit the virus.

A field hospital for treating Covid-19 patients outside Port Elizabeth, South Africa, in November. Credit…Samantha Reinders for The New York Times

The key mutation, called E484K, and two of its companions alter the shape of a part of the virus that is crucial for immune recognition, making it difficult for antibodies to attach themselves to the virus. The trio popped up in several lab studies that tried to predict which mutations would be advantageous to the virus.

“I think we need to monitor mutations closely and look out for things like that that could be becoming dominant in certain parts of the world,” said Akiko Iwasaki, an immunologist at Yale University.

Britain detected the more contagious variant circulating there because it sequences more virus samples than any other nation. The United States lags far behind: It has sequenced about 71,000 samples so far, a tiny proportion of the millions infected in the country. But the Centers for Disease Control and Prevention plans to work with state and local public health labs to sequence as many as 6,000 samples per week, agency scientists said Friday.

It will be important to limit travel — and the import of variants — from other countries until a majority of the population is immunized, said John Moore, a virologist at Weill Cornell Medicine in New York.

“Even if they are already here, the more often they are reintroduced, the more likely there could be a super-spreader event,” Dr. Moore said. (President Joseph R. Biden Jr. plans to sustain existing travel restrictions on anyone who has recently traveled to Europe and Brazil.)

The mRNA technology on which the Pfizer and Moderna vaccines rely can be altered in a matter of weeks, and far more easily than the process used to produce flu vaccines. But it would be wise to prepare for this eventuality now and think through not just the technical aspects of updating the vaccines, but the testing, approval and rollout of those vaccines, experts said.

Still, the best path forward is to prevent the emergence of new mutations and variants altogether, they said.

“Imagine having to do catch-up like this all the time — it’s not something desirable,” Dr. Iwasaki said. “If we can just stop the spread as soon as possible, while the vaccine is very effective, that’s the best way.”

Now That Grandma Has Been Vaccinated, May I Visit Her?

Now That Grandma Has Been Vaccinated, May I Visit Her?

The start of a mass coronavirus vaccination campaign at U.S. nursing homes has brought hope to many families. But it may be a while before restrictions loosen. Here are answers to common questions.

Vera Leip, 88, of Pompano Beach, Fla., received the Pfizer-BioNtech vaccine at the John Knox Village retirement community last week.
Vera Leip, 88, of Pompano Beach, Fla., received the Pfizer-BioNtech vaccine at the John Knox Village retirement community last week.Credit…Joe Raedle/Getty Images

  • Dec. 21, 2020, 4:48 p.m. ET

A watershed moment has arrived for many families: This week health care workers from CVS and Walgreens, under contract from the federal government, will fan out to nursing homes across the country to begin vaccinating residents against the coronavirus. The shots not only will help protect the nation’s elderly and infirm — and the staff who care for them — but they raise the prospect of ending the devastating isolation many residents have felt for months.

Family members are hopeful that before too long, they will return to visiting parents and grandparents, aunts, uncles and other loved ones regularly again. We checked with experts on some common questions.

Will restrictions on visiting be lifted soon?

Probably not in a big way. Restrictions vary by state, and the federal government’s guidance on what it considers safe stands for now. They already allow visits under certain conditions. The Centers for Medicare & Medicaid Services, or C.M.S., recommended in September that outdoor visits with residents be allowed and indoor visits, too, if the facility has been free of cases for 14 days.

Some medical experts have said that those guidelines are too lax and that visits should be severely restricted, even banned. However, some of these experts are now saying that the vaccine changes the equation, somewhat.

“Once all residents are vaccinated, it opens the door for loosening of restrictions,” said Dr. Michael Wasserman, the immediate past president of the California Association of Long Term Care Medicine, a geriatrician and former executive at nursing home chains.

To allow visits, Dr. Wasserman recommends all residents of a nursing home should be vaccinated (unless they have some condition or allergy that would discourage vaccination on medical grounds); all staff members should be vaccinated; the nursing home should have the ability to ensure that visitors test negative for the coronavirus and have been disciplined about wearing a mask in public settings.

Is the vaccine safe and effective for old and frail residents of nursing homes?

The clinical trials of the Pfizer and Moderna vaccine included people over 65, and results showed it to be safe and to work as well in older people as in younger ones.

“This vaccine has gone through testing and clinical trials to ensure it meets the highest safety standards. It also is safe to get if you already had the virus,” says a campaign to encourage people to get the shots by the American Health Care Association and National Center for Assisted Living, a combined trade group representing nursing homes and assisted-living communities.

The lead administrator for C.M.S., Seema Verma, reinforced the confidence in the shot for older patients, including those with health conditions, in a statement last week: “I urge states to prioritize nursing homes and vulnerable seniors in their distribution of the vaccine.”

The point is echoed by Dr. Sabine von Preyss-Friedman, chief medical officer of Avalon Health Care Group, which operates nursing homes, who said the new vaccines appear “safe and effective.”

If restrictions are eased, should I visit right away?

The Pfizer and Moderna vaccines both require two injections — the initial shot and a booster three or four weeks later. Dr. von Preyss-Friedman recommends waiting at least two weeks after the second shot to have a visit.

“You hope these vaccines work, but these are elderly patients,” she said. “You want to err on the side of protection.”

She said that, ideally, the visitor would also be vaccinated as well. Since shots won’t be widely available for a few months, it may be best to wait until you get your vaccine. Until then, she believes nursing homes should consider visits on a case-by-case basis.

Would visitors still need to wear a mask?

Absolutely, medical experts said. This is particularly true if they are not vaccinated, but even after they are vaccinated “until rates in the community go down,” said Dr. Joshua Uy, a geriatrician and associate professor at the University of Pennsylvania Medical School and the medical director of Renaissance Healthcare & Rehabilitation Center, a nursing home in Philadelphia.

Dr. Uy said that he hopes that the federal government would supply enough personal protective equipment so that all visitors and residents could be properly gowned for such visits.

What is being done to encourage nursing home residents to get vaccinated?

The combined nursing-home and assisted-living trade group has started a program aimed at helping nursing homes and other care facilities to explain to residents the essential need to get the vaccine. The campaign, #getvaccinated, notes: “The elderly population has a much higher risk for getting very sick, being hospitalized, or dying from Covid-19. The vaccine has been shown to provide a great deal of protection against serious illness due to Covid-19.”

But the people they love most may have more effective persuasive powers. Families can help, Dr. Uy said, by encouraging their parents and grandparents in nursing homes to get vaccinated.

“The vaccine,” he said, “is going to be our way out.”

Frontline Workers and People Over 74 Should Get Shots Next, CDC Panel Says

Frontline Workers and People Over 74 Should Get Shots Next, CDC Panel Says

The recommendation was a compromise aimed at getting the coronavirus vaccine to the most vulnerable of two high-risk groups.

Director of nursing education Loraine Hopkins Pepe, left, administers the Pfizer BioNTech COVID-19 vaccine to Dr. Richard Fine, head of anesthesiology, at Einstein Medical Center in Philadelphia, PA.
Director of nursing education Loraine Hopkins Pepe, left, administers the Pfizer BioNTech COVID-19 vaccine to Dr. Richard Fine, head of anesthesiology, at Einstein Medical Center in Philadelphia, PA.Credit…Hannah Yoon for The New York Times
  • Dec. 20, 2020, 3:40 p.m. ET

Striking a compromise between two high-risk population groups, a panel advising the Centers for Disease Control and Prevention voted Sunday to recommend that people 75 and older be next in line to receive the coronavirus vaccine in the United States, along with about 30 million “frontline essential workers,” including emergency responders, teachers and grocery store employees.

The debate about who should receive the vaccine in these early months has grown increasingly urgent as the daily tally of cases has swelled to numbers unimaginable even a month ago. The country has already begun vaccinating health care workers, and on Monday, CVS and Walgreens were to begin a mass vaccination campaign at the nation’s nursing homes and long-term care facilities. This week roughly six million doses of the newly authorized Moderna vaccine are to start arriving at more than 3,700 locations around the country, including many smaller and rural hospitals.

The panel of doctors and public health experts had previously indicated it would recommend a much broader group of Americans defined as essential workers — about 87 million people with jobs designated by a division of the Department of Homeland Security as critical to keeping society functioning — as the next priority population and that elderly people who live independently should come later.

But in hours of discussion on Sunday, the committee members concluded that given the limited initial supply of vaccine and the higher Covid-19 death rate among elderly Americans, it made more sense to allow the oldest among them to go next along with workers at the highest risk of exposure to the virus.

Groups of essential workers, such as construction and food service workers, the committee said, would be eligible for the next wave. Members did clarify that local organizations had great flexibility to make those determinations.

“I feel very strongly we do need to have that balance of saving lives and keeping our infrastructure in place,” said Dr. Helen Talbot, a member of the panel and infectious-disease specialist at Vanderbilt University.

Together, the two groups the committee voted to prioritize on Sunday number about 51 million people; federal health officials have estimated that there should be enough vaccine supply to inoculate all of them before the end of February.

Still, as the first week of vaccinations in the U.S. came to a close, frustrations were flaring about the pace of distribution. Some 128,000 shots had been given in the as of Friday, according to a New York Times database tracking vaccinations — a total that was just slightly more than half the number of new cases reported across the country that same day.

This weekend Gen. Gustave F. Perna, who heads the Trump administration’s distribution effort, apologized for more than a dozen states learning at the last minute that they would receive fewer doses next week of the vaccine manufactured by Pfizer than they had expected. Tensions were also broiling in some states over local decisions regarding which health care workers should get their shots immediately and which should wait.

The director of the C.D.C., Dr. Robert Redfield, will review the panel’s recommendation and decide, likely by Monday, whether to embrace it as the agency’s official guidance to states. The panel, the Advisory Committee on Immunization Practices, emphasized that its recommendations were nonbinding and that every state would be able to fine-tune or adjust them to serve the unique needs of its population.

The 13-to-1 vote came as frustrations flared about the pace of vaccine distribution. Some 128,000 shots have been given in the first five days of the vaccine United States, according to a New York Times database tracking vaccinations — just slightly more than half the number of new cases reported across the country on Friday alone. This weekend, Gen. Gustave F. Perna, who heads the Trump administration’s distribution effort, apologized for at least 14 states learning at the last minute that they would receive fewer doses of the vaccine manufactured by Pfizer next week than they had expected. Tensions were also flaring in some states over local decisions regarding which health care workers should get their shots immediately, and which should wait.

In addition to teachers, firefighters and police, a subgroup of the committee suggested that “frontline essential workers” should include school support staff; day care, corrections personnel, public transit, grocery store and postal workers; and those in working in food production and manufacturing. But the group’s official recommendation is not that specific.

Originally, the committee had signaled last month that they had been inclined to let 87 million essential workers receive vaccines ahead of adults 65 and older. Many had expressed their alarm that essential workers, who are often low-wage people of color, were being hit disproportionately hard by the virus and additionally were disadvantaged because of their lesser access to good health care.

general population.”

Answers to Your Questions About the New Covid Vaccines in the U.S.

Answers to Your Questions About the New Covid Vaccines in the U.S.

Vaccines are rolling out to health workers now and will reach the arms of the rest of us by spring. Here’s what you need to know.

The Pfizer-BioNTech Covid vaccines are prepared to be shipped at a Pfizer plant on Dec. 13, 2020 in Portage, Michigan.
The Pfizer-BioNTech Covid vaccines are prepared to be shipped at a Pfizer plant on Dec. 13, 2020 in Portage, Michigan.Credit…Pool photo by Morry Gash
  • Dec. 14, 2020, 1:47 p.m. ET

Getting the vaccine

Why can’t everyone get the vaccine now?

There aren’t enough doses for everyone, so initially the vaccine will be rationed for those who need it most. It will take time to produce and distribute the vaccine, and then schedule two vaccinations per person, three to four weeks apart. As more vaccines get approved, things will speed up. At least 40 million doses (enough for 20 million people) should be available from Pfizer and Moderna by the end of the year, and much more will come in 2021. How many doses will your state get? Look up your state’s vaccine distribution plans here. —Abby Goodnough

Who will get the vaccine first?

Here’s the expected order for vaccinations:

  • Health care workers and people in long-term care facilities: The nation’s 21 million health care workers and three million mostly elderly people living in long-term care facilities will go first, starting in December. Initially, there won’t be enough doses to vaccinate all health care workers, so states will prioritize based on exposure risk, choosing emergency room staff, for instance, to go first. Or they may offer the vaccine to the oldest health care workers first.

  • Essential workers: The 87 million Americans who work in food and agriculture, manufacturing, law enforcement, education, transportation, corrections, emergency response and other sectors, likely will be second in line, starting early next year. States will set priorities. Arkansas, for example, has proposed including workers in its large poultry industry, while Colorado wants to include ski industry workers who live in congregate housing.

  • Adults with underlying medical conditions and people over 65. Health officials are hoping to get any remaining older adults who have not been vaccinated sometime in the first quarter. Some states might decide to vaccinate residents over 75 before some types of essential workers.

  • All other adults. Adults in the general population are at the back of the line. They could start receiving the vaccine as early as April, said Dr. Anthony S. Fauci, the nation’s top infectious disease expert, although many people likely will have to wait until at least May or June. The vaccine hasn’t been approved in children, so it may be several months, or possibly a year, before the vaccine is available for anyone under the age of 16. —Abby Goodnough, Tara Parker-Pope

How will the first doses of the vaccine get to health workers?

Hospitals and medical groups are contacting health workers to schedule vaccine appointments. FedEx and UPS will transport the vaccine throughout most of the country, and each delivery will be followed by shipments of extra dry ice a day later.

Pfizer designed special containers, with trackers and enough dry ice to keep the doses sufficiently cold for up to 10 days. Every truck carrying the containers will have a device that tracks its location, temperature, light exposure and motion. Pfizer will ship the special coolers, each containing at least 1,000 doses, directly to locations determined by each state’s governor. At first, almost all of those sites will probably be hospitals that have confirmed they can store shipments at minus 94 degrees Fahrenheit, as the Pfizer vaccine requires, or use them quickly. —Abby Goodnough

How will the vaccine get to nursing homes?

The pharmacy chains CVS and Walgreens have contracts with the federal government to send teams of pharmacists and support staff into thousands of long-term care facilities in the coming weeks to vaccinate all willing residents and staff members. CVS and Walgreens are both planning to administer their first vaccinations on Dec. 21.

More than 40,000 facilities have chosen to work with CVS. Nearly 35,000 picked Walgreens. Each U.S. state has already picked, or will soon pick, either the Pfizer or the Moderna vaccine for all of its long-term care facilities that will be working with the pharmacies. —Rebecca Robbins, Abby Goodnough

How will the rest of us get vaccinated?

It’s likely that when the general public starts getting vaccinated in April, shots will be scheduled through doctor’s offices, CVS, Walgreens and other pharmacies — the same way people get flu shots. However, final plans will depend on what other vaccines besides Pfizer’s and Moderna’s have been approved. —Abby Goodnough, Rebecca Robbins

Can I choose which vaccine I get?

This depends on a number of factors, including the supply in your area at the time you’re vaccinated and whether certain vaccines are found to be more effective in certain populations, such as older adults. At first, the only choice will be Pfizer’s vaccine, though Moderna’s could become available within weeks. —Abby Goodnough

How long will it take to work?

You won’t get the full protection from the Pfizer-BioNTech vaccine until about a week after the second dose, based on clinical trial data. The researchers found that the vaccine’s protection started to emerge about ten days after the first dose, but it only reached 52 percent efficacy, according to a report in the New England Journal of Medicine. A week after the second dose, the efficacy rose to 95 percent. Read more here. —Carl Zimmer, Noah Weiland

Safety and side effects

Will it hurt? What are the side effects?

The injection into your arm won’t feel different than any other vaccine, but the rate of short-lived side effects does appear higher than a flu shot. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. The side effects, which can resemble the symptoms of Covid-19, last about a day and appear more likely after the second dose. Early reports from vaccine trials suggest some people might need to take a day off from work because they feel lousy after receiving the second dose. In the Pfizer study, about half developed fatigue. Other side effects occurred in at least 25 to 33 percent of patients, sometimes more, including headaches, chills and muscle pain.

While these experiences aren’t pleasant, they are a good sign that your own immune system is mounting a potent response to the vaccine that will provide long-lasting immunity. —Abby Goodnough, Carl Zimmer

How do I know it’s safe?

Each company’s application to the F.D.A. includes two months of follow-up safety data from Phase 3 of clinical trials conducted by universities and other independent bodies. In that phase, tens of thousands of volunteers got a vaccine and waited to see if they became infected, compared with others who received a placebo. By September, Pfizer’s trial had 44,000 participants; no serious safety concerns have been reported. — Abby Goodnough

If I have allergies, should I be concerned?

People with severe allergies who have experienced anaphylaxis in the past should talk to their doctors about how to safely get the vaccine and what precautions to take. Although severe reactions to vaccines are rare, two health care workers had anaphylaxis after receiving the vaccine on the first day it became available in Britain. Both workers, who had a history of severe reactions, were treated and have recovered. (Anaphylaxis can be life-threatening, with impaired breathing and drops in blood pressure that usually occur within minutes or even seconds after exposure to a food, medicine or substance like latex.) For now, British authorities have said the vaccine should not be given to anyone who has ever had an anaphylactic reaction, but U.S. health experts have said such warnings are premature because severe reactions can be treated or prevented with medications. Because of the British cases, the F.D.A. said it would require Pfizer to increase its monitoring for anaphylaxis and submit data on it once the vaccine comes into use. Fewer than one in a million recipients of other vaccines a year in the U.S. have an anaphylactic reaction, said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia.

Among those who participated in the Pfizer trials, a very small number of people had allergic reactions. A document published by the F.D.A. said that 0.63 percent of participants who received the vaccine reported potential allergic reactions, compared to 0.51 percent of people who received a placebo. In Pfizer’s late-stage clinical trial, one of the 18,801 participants who received the vaccine had an anaphylactic reaction, according to safety data published by the F.D.A. on Tuesday. None in the placebo group did. Read more here. — Denise Grady

What about my situation? Answers about different types of patients.

I had Covid-19 already. Do I need the vaccine?

It’s safe, and probably even beneficial, for anyone who has had Covid to get the vaccine at some point, experts said. Although people who have contracted the virus do have immunity, it is too soon to know how long it lasts. So for now, it makes sense for them to get the shot. The question is when. Some members of the C.D.C. advisory committee have suggested people who have had Covid in the past 90 days should be toward the back of the line.Read more here. —Abby Goodnough, Apoorva Mandavilli

Will it work on older people?

All the evidence we have so far suggests that the answer is yes. The clinical trials for the two leading vaccines have shown that they work about the same in older people as younger people. As the vaccines get distributed, the vaccine makers and the C.D.C. will continue to monitor the effectiveness of the vaccine in people 65 and older who, because of age-related changes in their immune systems, often don’t respond as well to vaccination as younger people do. But just as certain flu vaccines have been developed to evoke a stronger immune response in older people, it’s possible that one of the new vaccines could emerge as a better option for this age group. It’s just far too soon to know. —Carl Zimmer

I’m young and at low risk. Why not take my chances with Covid-19 rather than get a vaccine?

Covid-19 is by far the more dangerous option. Although people who are older, obese or have other health problems are at highest risk for complications from Covid-19, younger people can become severely ill, too. In a study of more than 3,000 people ages 18 to 34 who were hospitalized for Covid, 20 percent required intensive care and 3 percent died.

And as many as one in three people who recover from Covid have chronic complaints, including exhaustion, a racing heart and worse for months afterward. Covid vaccines, in contrast, carry little known risk. Read more here. —Apoorva Mandavilli

Vaccinating pregnant women and children

What about women who are pregnant or breastfeeding?

Pregnant and breastfeeding women should consult with their obstetricians and pediatricians about whether to get the vaccine. The Pfizer vaccine has not been tested in pregnant women or in those who were breastfeeding, and federal health officials have not issued any specific guidance, other than allowing these women to be vaccinated if they choose. (The American College of Obstetricians and Gynecologists issued practice guidelines to help women and their doctors talk about vaccination.)

In the initial rollout, it will be mostly pregnant health care workers who must weigh the benefits and possible risks. By the time the vaccine is available to pregnant essential workers or to women in the general population, there should be a lot more data available.

Some experts said the virus itself poses greater risks to pregnant women than the new vaccine. Since the 1960s, pregnant women have been urged to receive vaccines against influenza and other diseases. These women are generally cautioned against live vaccines, which contain weakened pathogens — but the Pfizer vaccine does not contain live virus. Read more here.Apoorva Mandavilli

Does the vaccine affect fertility or miscarriage risk?

A false claim has been circulating online that the new vaccine will threaten women’s fertility by harming the placenta. Here’s why it’s not true.

The claim stems from the fact that the vaccines from Pfizer and Moderna cause our immune systems to make antibodies to something called a “spike” protein on the coronavirus. The false warnings about fertility are based on the claim that these antibodies could also attack a similar protein that is made in the placenta during pregnancy, called syncytin. In reality, the spike protein and syncytin are similar only in one very small region, and there’s no reason to believe antibodies that can grab onto spike proteins would lock onto syncytin.

What’s more, the human body generates its own supply of spike antibodies when it fights off the coronavirus, and there’s no sign that these antibodies attack the placenta. If they did, you’d expect that women who got Covid-19 would suffer miscarriages. But a number of studies show that Covid-19 does not trigger miscarriages. Read more here. —Carl Zimmer

When will vaccines be available for children?

So far, no coronavirus vaccine has been approved for children. New vaccines are typically tested on adults before researchers launch trials on children, and coronavirus vaccine developers are following this protocol. In September, Pfizer and BioNTech began studying their vaccine on children as young as 12. Moderna followed suit in December. If these trials yield good results, the companies will recruit younger children. The FDA will then have to review these results before the vaccines can get emergency authorization. Read more here.—Carl Zimmer

Why weren’t children included in the early studies?

Vaccines are typically tested on adults first in the interest of safety. But once a vaccine is shown to be safe and effective in adults, researchers have to run more trials on children to adjust the dosage for their bodies. Another factor in the wait for a vaccine for children is that they are far less likely to die from Covid-19 than adults are. The Centers for Disease Control and Prevention issued a report in September which concluded that, of more than 190,000 people who died in the United States with Covid-19, only 121 were under the age of 21. —Carl Zimmer and Katie Thomas

Life after vaccination

What if I forget to take the second dose on time?

Both the vaccines from Pfizer-BioNTech and from Moderna have two doses, with the booster shot coming a few weeks after the first. Pfizer-BioNTech’s second dose comes three weeks after the first, and Moderna’s comes four weeks later. The second dose provides a potent boost that gives people strong, long-lasting immunity.

If for some reason you fail to get the second shot precisely three weeks after the first, you don’t have to start all over again with another two-dose regimen. “The second dose can be picked up at any time after the first. No need to start the series over,” said Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the F.D.A.’s vaccine advisory panel.

And while the two leading vaccines include a second dose, some future vaccine candidates may only require one dose. Johnson & Johnson, for example, is expecting data in January that will show whether its experimental vaccine works after a single dose. In case it doesn’t, the company has also started a separate trial using two doses. —Carl Zimmer, Tara Parker-Pope

If I’ve been vaccinated, will I still need to wear a mask?

Yes, but not forever. Here’s why. The coronavirus vaccines are injected deep into the muscles and stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill. But what’s not clear is whether it’s possible for the virus to bloom in the nose — and be sneezed or breathed out to infect others — even as antibodies elsewhere in the body have mobilized to prevent the vaccinated person from getting sick.

The vaccine clinical trials were designed to determine whether vaccinated people are protected from illness — not to find out whether they could still spread the coronavirus. Based on studies of flu vaccine and even patients infected with Covid-19, researchers have reason to be hopeful that vaccinated people won’t spread the virus, but more research is needed. In the meantime, everyone — even vaccinated people — will need to think of themselves as possible silent spreaders and keep wearing a mask. Read more here. —Apoorva Mandavilli

Will my employer require vaccinations?

Employers do have the right to compel their workers to be vaccinated once a vaccine is formally approved. Many hospital systems, for example, require annual flu shots. But employees can seek exemptions based on medical reasons or religious beliefs. In such cases, employers are supposed to provide a “reasonable accommodation” — with a coronavirus vaccine, for example, a worker might be allowed to work if they wear a mask, or to work from home. —Abby Goodnough

How will we know when things are getting better?

The test positivity rate in your community will be an indicator of how things are going. This number is the percentage of overall tests given in a community that come back positive. The lower the number, the fewer new cases and the less likely you are to cross paths with someone who has the virus. “The best number is zero,” Dr. Fauci said. “It’s never going to be zero, but anywhere close to that is great.” —Tara Parker-Pope

When can we start safely doing normal things, like going to the movies or the theater?

Public health officials estimate that 70 to 75 percent of the population needs to be vaccinated before people can start moving freely in society again. If things go well, life could get a lot better by late spring and early summer. “It depends on the uptake of the vaccine and the level of infection in the community,” Dr. Fauci said.

Given the surveys so far showing significant public reluctance to get vaccinated, however, it may take awhile to see widespread community protection, he said: “If it turns out that only 50 percent get vaccinated, then it’s going to take much, much longer to get back to the kind of normality that we’d like to see.” —Tara Parker-Pope

Will these vaccines put a dent in the epidemic?

The coronavirus vaccines will be much less effective at preventing death and illness in 2021 if they are introduced into a population where the virus is raging — as is now the case in the U.S. A vaccine that’s 95 percent effective, as Moderna’s and Pfizer’s versions appear to be, is a powerful fire hose. But the size of a fire is still a bigger determinant of how much destruction occurs.

According to the authors of a paper in the journal Health Affairs, at the current level of infection in the U.S. (about 200,000 confirmed new infections per day), a vaccine that is 95 percent effective — distributed at the expected pace — would still not be enough to end the terrible toll of the virus in the six months after it was introduced. Almost 10 million or so Americans would still contract the virus, and more than 160,000 would die.

Measures that reduce the virus’s spread — like mask-wearing, social distancing and rapid-result testing — can still have profound effects. Public health officials hope that people will continue to take these precautions at least until the country reaches a vaccination rate of 70 to 75 percent. —David Leonhardt

Will I be required to provide proof of vaccination to travel?

In the coming weeks, major airlines including United, JetBlue and Lufthansa plan to introduce a health passport app, called CommonPass, that aims to verify passengers’ coronavirus test results — and perhaps soon, vaccinations. CommonPass notifies users of local travel rules — like having to provide proof of a negative virus test — and then aims to check that they have met them.

Although no plans are in place yet to require proof of vaccination for travel or other activities, electronic vaccination credentials could have a profound effect on efforts to control the virus and restore the economy. They could prompt more employers and college campuses to reopen. And developers say they may also give some consumers peace of mind by creating an easy way for movie theaters, cruise ships and sports arenas to admit only those with documented virus vaccinations. Read the full story. —Natasha Singer

How long will the vaccine last? Will I need another one next year?

That is to be determined. It’s possible that coronavirus vaccinations will become an annual event, just like the flu shot. Or it may be that the benefits of the vaccine last longer than a year. We have to wait to see how durable the protection from the vaccines is. Immunity from coronavirus infections appears to last for months, at least, so that may be a hint about vaccines. —Carl Zimmer

How the different vaccines work

How do these new genetic vaccines work?

The Pfizer-BioNTech and Moderna vaccines use a genetic molecule to prime the immune system. That molecule, known as mRNA, is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. While the immune protection from these vaccines may last for months or perhaps even years, their mRNA does not — it is destroyed by our cells within days. Although these are described as “genetic” vaccines, the vaccines don’t alter your genes in any way. —Carl Zimmer

What do the vaccine developers mean when they say their vaccines are 95 percent effective?

Vaccine developers test their vaccines in clinical trials. The fundamental logic behind these trials was worked out by statisticians over a century ago. Researchers vaccinate some people and give a placebo to others. They then wait for participants to get sick and look at how many of the illnesses came from each group.

In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to come down with symptoms of Covid-19 and then get a positive test. Among those who got sick, 162 had received a placebo shot, and just eight had received the real vaccine. This result shows that receiving a vaccine dramatically lowered the chances of getting Covid-19 compared to receiving a placebo.

The difference is expressed as efficacy: the Pfizer-BioNTech vaccine has an efficacy rate of 95 percent. (If there were no difference between the vaccine and placebo groups, the efficacy would be zero. If none of the sick people had been vaccinated, the efficacy rate would have been 100 percent.) A vaccine’s efficacy rate and effectiveness rate are different: Efficacy is a measurement made within the strict confines of a clinical trial, whereas effectiveness refers to how a vaccine works in the real world. No one knows yet the true effectiveness of these new vaccines. —Carl Zimmer

Was the Pfizer vaccine part of the government’s Operation Warp Speed?

Pfizer did not accept federal funding to help develop or manufacture the vaccine, unlike front-runners Moderna and AstraZeneca. Pfizer did get a $1.95 billion deal with the government to deliver 100 million doses of the vaccine. The arrangement is an advance-purchase agreement, meaning that the company won’t get paid until they deliver the vaccines. Read more here. —Carl Zimmer and Katie Thomas

What does the rollout of the Pfizer vaccine mean for the other vaccines in the race?

Researchers were heartened by the strong results of the vaccine developed by Pfizer and BioNTech. “It gives us more hope that other vaccines are going to be effective too,” said Akiko Iwasaki of Yale University.

The Moderna vaccine, which is next in line for approval, has an efficacy rate of 94.5 percent, essentially the same as the Pfizer-BioNTech vaccine. A vaccine from AstraZeneca and the University of Oxford has shown confusing trial results, with efficacy rates between 60 and 90 percent, depending on the strength of the doses that volunteers received. And the French company Sanofi had a major disappointment in its early clinical trials, finding that its vaccine couldn’t provoke an immune response in people over 55. The company is now reformulating its vaccine to start new trials. —Carl Zimmer and Katie Thomas

Reporting contributed by:

Abby Goodnough, Carl Zimmer, Rebecca Robbins, Apoorva Mandavilli, Denise Grady Katie Thomas, Tara Parker-Pope, Noah Weiland, Natasha Singer, David Leonhardt, Roni Caryn Rabin, Julie Bosman, Reed Abelson and Richard Pérez-Peña