Tag: Pfizer Inc

C.D.C. Panel Recommends Covid Booster Shots for Millions of Americans

Recipients of the Moderna and the J.&J. vaccines may receive extra doses, the committee said, although the shots continue to prevent illness and death.

Data from Federal Scientists Raise Questions About J.&J. Booster Shots

People who have received the company’s one-shot vaccine may benefit from a booster with another brand. F.D.A. advisers will discuss the data on Friday.

Delta Variant Widens Gulf Between ‘Two Americas’: Vaccinated and Unvaccinated

Data from overseas, particularly Britain, suggest the spread of the virus will set vaccinated and unvaccinated communities on very different paths.

What the C.E.O. of Pfizer and Other Covid-19 Fighters Learned From Their Parents

Various people working to stop the pandemic reflected on the life skills their parents taught them: determination, teamwork, resilience and more.

Debra Weiner interviewed people who are working to beat the coronavirus about the most valuable things their parents taught them. Following are excerpts from a few of those stories, edited and condensed.

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Anything Is Possible

Albert Bourla Chairman and chief executive officer of Pfizer

Albert Bourla, third from left, at his wedding with, from left, his father-in-law, Jack Alchanati; his mother, Sara Bourla; his wife, Myriam Alchanati; his mother-in-law, Rachel Alchanati; his father, Mois Bourla. 
Albert Bourla, third from left, at his wedding with, from left, his father-in-law, Jack Alchanati; his mother, Sara Bourla; his wife, Myriam Alchanati; his mother-in-law, Rachel Alchanati; his father, Mois Bourla. via Albert Bourla

I was born in Thessaloniki, Greece. Before the Holocaust, there were 50,000 Jews there. About 95 percent were exterminated. A lot of survivors don’t discuss what happened to them. My family did. On Sundays, we would gather in the living room with the relatives. My sister and cousins and I would be sitting on the floor and we’d say: “Tell us a story about that time. Tell us a story.” And my mother would.

My mother was the youngest of seven children. She lived in hiding with her oldest sister for a year, starting in 1943. Like Anne Frank, my mom wasn’t supposed to go out of the house. But she was a teenager, didn’t follow all the rules, and one day when she was out, she was spotted and arrested and put in prison.

This was toward the end of the war and the Germans were no longer sending the Jews in Greece to Auschwitz. But as a prisoner, she was beaten and abused. And every day at noon, some of the prisoners would be taken to the other side of town and the next day executed. Her sister’s husband, who was quite wealthy, had paid a ransom to the commander of the Nazi occupation in Thessaloniki. So her sister thought my mother was secure. Still, each day she would go to the prison to see who was going out. And one day she saw my mom loaded into the truck. Her sister ran to tell her husband, who contacted the commander. “I gave you all this money. What’s this?”

He said, “I have no idea what you’re talking about. Let me see.”

My mother didn’t sleep at all that night. Someone told her to be brave but she just kept crying. At dawn, she and the others were lined up against a wall in front of a firing squad when at the last moment, a BMW motorbike with two German soldiers drove up. They handed some documents to the officer in charge and my mother and another woman were removed from the line. As they left to go back to the prison, she could hear the machine guns slaughtering the others.

My mother told us about everything in detail with the same easiness that I share childhood memories with my kids. She never said, “Look what the Germans did to me.” That was irrelevant. And she never said, “Oh, when I suffered.” She put a humorous spin on things so we didn’t feel the horror. And most important, the stories were filled with messages of optimism: “I was in worst position once, and now I have you and your sister. Life is miraculous. Nothing is impossible.”

That was the spirit of her. And she inspired me to be the same. In my first year of middle school, they told us there were going to be elections for class president, vice president and secretary. I asked my mom, “Do you think I should raise my hand?”

She said yes.

“But I’m the only Jew in the school.”

“Just do it. Go make your speech.”

And I was elected president.

My mother believed you can do anything in life. That there’s always a way. The way may not be clear in the beginning, but there is always a way. I owe her a lot because of that. She is my role model.

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Listen to the Other Side

Dr. Soumya Swaminathan Chief scientist at the World Health Organization, known for her research on tuberculosis and H.I.V.

Soumya Swaminathan, far right, with her parents, Mina and MS Swaminathan, and her sisters, Nitya, front left, and Madhura, center.via Soumya Swaminathan

My father, MS Swaminathan, shot into prominence when he was very young. He collaborated with the Nobel Prize winner Norman Borlaug and developed new, high-yielding varieties of seeds for wheat and rice, and convinced farmers around Delhi and Punjab to grow them. Wheat production went up three or four times. From being a nation that had to import food grains from the United States, by the early 1970s, we were basically food secure. My father became known as the Father of the Green Revolution.

But at the peak of his career, there was an attack on his work by some of his closest colleagues about the unintended side-effects of improving the yields of plants. My father had himself recognized that because of the use of pesticides and fertilizers there would be some environmental damage and contamination of the water, and he had spoken about it. But there was a euphoria at the time that India had become food self-sufficient. It was immediate benefits versus long-term risks.

The criticism was in all the newspapers. Kids at school would ask, “Is what they said true? Did your father do these bad things?” And the atmosphere at home was somber. I remember asking my father, “Don’t you hate these people who write all these nasty things about you?”

“No, no, no. I don’t hate them,” he said. “There’s no point in being angry. It’s their right to question and write what they want. If you believe in something and think you are doing the right thing, then even if people criticize you, you carry on. It can seem unfair, but there could also be some something you can take from it, some element you didn’t do right or didn’t communicate in the right way that you can try to improve on.”

My father was a problem solver and he did it by listening to the people who were most affected by the problem. On weekends or holidays, my sisters and I, and sometimes our family’s gardener’s kids, would often go with him to the farming villages. While we were running around in the sugar cane and wheat fields playing hide and seek, he would sit with the farmers and hear how they were doing with the new seeds and if they were having any problems, and be open to changing course if needed. He would always say to the farmers, “This will only be a success story if it works for you.”

As a young tuberculosis researcher, I went into a remote tribal community in south India to explain why it’s very important to identify and treat people with TB early. People in the tribe said to me: “Maybe once in two years somebody in our village gets diagnosed with TB. Our people die of other infectious diseases. Children die from diarrhea. Somebody falls in the forest, fractures their leg and we have to carry that person 15 kilometers to a health center where most often there’s no surgeon so nothing can be done. These are our day-to-day problems, so it doesn’t seem appropriate that you’re discussing TB but aren’t trying to solve our other issues.” I thought then of my childhood and what I saw my father do. He knew that you needed to look holistically at people’s physical, social and economic environment and see through their eyes.

When I am in a situation where I’m in disagreement or have a completely different view, I may feel at the moment quite upset. But when I reflect further or have a discussion and try to understand where the other person is coming from, I start to say, “OK, this is why they are so negative or angry.” And that is when you start getting solutions.

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Fierce Determination

Michelle Gaskill-Hames Senior vice president for hospital and health plan operations for Kaiser Permanente’s Northern California region

Michelle Gaskill-Hames with her father, Ivan Gaskill.via Michelle Gaskill-Hames

My parents were born and raised in the same working-class neighborhood in Detroit. They played together as kids, dated in high school, then both went to Wayne State University. My mother majored in special education. My father had a math and computer science degree and was selected for a management training program at Michigan Bell, which was unusual for a young African-American in the early 1960s. Then, within less than a year of getting married, he was driving home from work and a car went through a red light and slammed into his.

His car was literally wrapped around a tree. They saved his life, but he was left paralyzed from the waist down with limited upper body movement. He was classified as a quadriplegic. He was 23. My mother was 22.

Back in that day, many quadriplegics gave up. That was never my father’s story. He wasn’t going to let the accident determine his life. He was ambitious. He had goals. And with my mother right there with him, nothing was going to deter him from returning to work, getting a house and adopting me. It’s funny. Some people might think having a father who was a quadriplegic is not a blessing. But to have parents who lived with such determination was the best blessing I could have ever had.

As a child, I thought it was cool that he had an electric wheelchair. I’d ride on his lap through the neighborhood to go get ice cream. When I got bigger, I’d ride my bike and he’d zoom along with me. I didn’t realize that people with disabilities could be discriminated against until we were at a restaurant once when the waiter came to the table, looked at my mother and said, “What would he like to order?”

My father didn’t raise his voice. He didn’t bat an eye. He just said, “Well, I will have the prime rib, medium-rare, horseradish on the side,” and made it very clear that his disability was not in any way connected to his mind.

That was my first recognition that people might feel sorry for my father or look down on him in some way. But I never did. And my dad never showed any sign of self-pity. He believed there was nothing you couldn’t do if you were focused and determined.

It’s not that there weren’t obstacles, but he believed that obstacles were meant to be overcome.

When I was in sixth grade, there was an oratory contest. Everyone had to recite a poem or speech. I thought I’d do the “I Have a Dream” speech. But my father said, “No, everybody is going to do that,” and he suggested the poem “If” by Rudyard Kipling.

It’s been my mantra ever since. I have a framed copy of it at work, right behind my desk. Whenever I’m dealing with a stressful situation, I turn around and read it. Even if I just get through the first lines: If you can keep your head when all about you / Are losing theirs and blaming it on you / If you can trust yourself when all men doubt you / But make allowance for their doubting too; I’m like, “OK. Got it. I can do this.” Not just because the words of the poem instill confidence, but because my father taught it to me. He was my hero. In my mind, he was 7 feet tall.

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Resilience

Ilona Bartnik Critical care nurse, University of Chicago Medical Center

Ilona Bartnik as a baby with, from left, her grandmother Jozefa Bartnik; her mother, Renata Bartnik, and her father, Krzysztof Bartnik.via Ilona Bartnik

I came to Chicago from Poland in 2002. My first day in school someone gave me a list of my classes and room numbers, but I didn’t know which way to go. I didn’t know who to ask or how to ask, because I spoke then a very broken English. I was afraid people wouldn’t understand me or that I wouldn’t understand them, and then they’d make fun of me.

Especially in the beginning, I’d come home from school crying. Not because anyone had been mean, but because I was overwhelmed. My parents helped as much as they could. They’d say the typical things you tell a 17-year-old girl: “Oh, it’ll get better. You’ll make new friends.” Then they would make me my favorite meal — pierogies with strawberries or blueberries.

But I would say: “I miss my old friends. I wish we could go back to Poland. I don’t know what we’re even doing here.”

There, my mom owned a small grocery store. Everyone in our town knew us. Here, my mom was a cleaning lady. It was embarrassing. Later my mom studied to become a dialysis technician. She would work long hours then go to school. On one hand, OK, now I don’t have to say she cleans houses. But then I never got to see her much because she was so busy.

But if it was hard for me to basically learn everything again, for my parents it was much harder. Learning a new language, getting a house in a good area, applying for a loan, even going to a grocery store, all of that was complicated. I’d hear them talking to their Polish friends who’d been here longer about which neighborhoods had good schools. Then they’d do the math to see what they could afford and get an extra job on the side just to make sure.

So they did whatever they needed to do to support us. But culture-wise, they were not as adaptable. They’d introduce me to the kids of their Polish friends, but my closest friends were other nationalities: Russian, Hispanic, Bulgarian, German. My parents were nice to them in their presence, but given Polish history there is a bit of tension between Polish and Germans, and Polish and Russians. And later it was always like, “Why don’t you just hang out with your own people” kind of thing. They didn’t understand that you can be a different color and still share the same things.

Maybe my parents preferred their own culture because it reminded them of home. Or because it felt like the one piece in their lives that they could control. But because my parents moved us here, I went from “I don’t want to change” to “I don’t want to stay the same.” I like trying new things.

This last year has been rough. Rules at the hospital changed so fast and so much. One week it was this; the next week it was that. Everyone was stressed out. But because of what I had to go through in experiencing a totally different culture, I knew I could adapt. I knew I was resilient. I knew I’d be able to cope.

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Personal Connection

Dr. Anne Schuchat Principal deputy director of the Centers for Disease Control and Prevention, retiring this month

Anne Schuchat, center, with her parents, Molly and Michael Schuchat.via Anne Schuchat

A lot of people are upset when they see bad things happening in the world. But my parents said, “What can we do about it?” When the Russians invaded Hungary in 1956, my parents, who’d only been married three years, had two kids and were expecting a third, opened their home for several months to a Hungarian refugee family that had escaped. Later, with others from her synagogue, my mom helped set up the Anne Frank House, a residence for homeless women in Washington.

I wouldn’t say they were activists, but life was about connecting with people and our home was a gathering place. After having five kids in seven years, my mom went back to school to get her Ph.D. in anthropology, and she would host the department parties. This was in the ’60s, so you can imagine what her classmates were like. Later my mom became interested in China and got involved with the U.S. China Peoples Friendship Association, and all these folks who had lived there in the 1930s or ’40s, would come to our house for meetings. And Passover Seders were an event. We’d have almost 40 people both nights, and not just extended family. There’d be Jews and non-Jews, friends from school who’d never been to a Seder, my mom’s professors, people visiting from other countries.

Bringing people together was part of my mom’s route to happiness, and that’s maybe why I try to turn whatever community I’m with into a welcoming environment. At the C.D.C., we have these two-year trainings for epidemic intelligence service officers. When I was branch chief, I remembered how in my mom’s office there was this fake certificate from a behavioral services organization she worked for that read, “This officially honors Molly Schuchat,” then said all these silly things. So I started making these personal little collages for the officers when they graduated. For the person who did nose swabs on 4,000 people, the border was lots of little noses. The one who’d negotiated lots of complicated relationships between institutions got the World Peace award.

In 2014, I was asked to lead this trial of an experimental Ebola vaccine in Sierra Leone. It was in the middle of an epidemic, working with counterparts who had lost friends to this virus. At times it seemed totally hopeless and unsolvable. Everybody was exhausted, hitting walls and a little bit scared. Somebody earlier had given me pompoms, so I pulled them out and went, “OK, we can get through this; this is what we’re going to do next,” and tried to help our team see that what they were doing was really brave, and that they were making a difference.

Public health can’t just be about the results because for every problem you solve, there are 10 more you have to tackle. It’s Sisyphean work. I’ve been lucky to be part of many things that have had a positive impact on the world. But as I moved into more senior positions, it was the warmth and connection I felt with the people I worked with that has given me joy. As my mom once said, “It doesn’t count if you don’t share it.”

Why the C.D.C. Changed Its Advice on Masks

Two scientific findings altered the calculus: Vaccinated people don’t transmit the virus, and the shots are effective against variants.

Advice from federal health officials that fully vaccinated people could drop their masks in most settings came as a surprise to Americans, from state officials to scientific experts. Even the White House got less than a day’s notice from the Centers for Disease Control and Prevention, the press secretary, Jen Psaki, said at a news briefing on Friday.

“The C.D.C., the doctors and medical experts there, are the ones who determined what this guidance would be based on their own data, and what the timeline would be,” Ms. Psaki said. “That was not a decision directed by or made by the White House.”

For months, federal officials have vigorously warned that wearing masks and social distancing were necessary to contain the pandemic. So what changed?

Introducing the new recommendations on Thursday, Dr. Rochelle P. Walensky, the C.D.C. director, cited two recent scientific findings as significant factors: Few vaccinated people become infected with the virus, and transmission seems rarer still; and the vaccines appear to be effective against all known variants of the coronavirus.

There is no doubt at this point that the vaccines are powerful. On Friday, the C.D.C. released results from another large study showing that the vaccines made by Pfizer-BioNTech and Moderna are 94 percent effective in preventing symptomatic illness in those who were fully vaccinated, and 82 percent effective even in those only partly vaccinated.

“The science is quite clear on this,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County. Mounting evidence indicates that people who are vaccinated are highly unlikely to catch or transmit the virus, she noted.

The risk “is definitely not zero, but it’s clear that it’s very low,” she said.

One of the lingering concerns among scientists had been that even a vaccinated person might carry the virus — perhaps briefly, without symptoms — and spread it to others. But C.D.C. research, including the new study, has consistently found few infections among those who received the Pfizer-BioNTech and Moderna vaccines.

“This study, added to the many studies that preceded it, was pivotal to C.D.C. changing its recommendations for those who are fully vaccinated against Covid-19,” Dr. Walensky said in a statement on Friday.

Other recent studies confirm that people who are infected after vaccination carry too little virus to infect others, said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai.

“It’s really hard to even sequence the virus sometimes because there’s very little virus, and it’s there for a short period of time,” he said.

Still, most of the data has been gathered on the Pfizer-BioNTech and Moderna vaccines, Dr. Krammer cautioned. Because Johnson & Johnson’s vaccine was authorized later, there are fewer studies assessing its effectiveness.

In clinical trials, the Johnson & Johnson vaccine had 72 percent efficacy — lower than the figure for the Pfizer and Moderna vaccines. And effectiveness was measured in terms of moderate and severe disease, rather than mild disease.

“It’s a very good vaccine, and I’m sure it will save many, many, many lives,” Dr. Krammer said. “But we need more data on how well the J.&J. vaccine prevents infection, and how well it prevents transmission.”

Variants of the virus have been a particular worry for scientists. While Dr. Walensky cited evidence showing that the mRNA vaccines like those from Pfizer and Moderna are effective against the variants circulating in the United States, there is little data about variants and the Johnson & Johnson vaccine. And new variants are emerging constantly.

“I’m not at all saying that this is now a big problem,” Dr. Krammer said. But before lifting the masking requirements, “I might have waited a little bit longer to look at the numbers.”

In a statement on Friday, a C.D.C. spokesman said, “All of the authorized vaccines provide strong protection against serious illness, hospitalization, and death, and we are accumulating data that our authorized vaccines are effective against the variants that are circulating in this country.”

Fully immunized people are unlikely to get seriously ill, even if they are infected with the coronavirus. The risk of infection is greater for the people around them — unvaccinated children and adults, or vaccinated people who remain unprotected because of a medical condition or treatment.

C.D.C. officials said they weighed those factors and were confident in their assessment of the science. And the new advice has other salutary effects, rewarding fully immunized people by giving them permission to end their social isolation — and perhaps incentivizing others to opt for vaccination.

The new advice “signals that we really are on the final stretch here, and I think that’s a very good thing for people,” said Dr. Joshua Sharfstein, the vice dean for public health practice and community engagement at Johns Hopkins University Bloomberg School of Health.

“It’s unlikely that we’re going to have another huge surge in cases,” he added. “But will the final stretch last for weeks or months is still a question.”

The difficulty with the new recommendations, he and other experts said, is not so much the science underpinning them as their implementation.

Leaders at the state, city and county levels still have the authority to require masks even for vaccinated people, as the C.D.C. was quick to acknowledge on Thursday. After the agency’s announcement, some states instantly lifted mask mandates, while others said they would need more time to weigh the evidence.

But in states without mask mandates, the onus of checking vaccination status will fall on shopkeepers, restaurant workers, school officials and workplace managers.

“Without a means to verify vaccination, we will have to rely on an honor system,” said Caitlin Rivers, an epidemiologist at Johns Hopkins University.

The number of cases in the country is the lowest it has been since September, and many experts support lifting mask mandates in much of the country. But doing so will be riskier in places like Michigan, where there are more cases, and for people who are unprotected, including children under 12 and people with a weak immune systems, Dr. Rivers said.

“People who are unvaccinated should continue to wear masks in public indoors and avoid crowds,” she said.

In Nacogdoches, Texas, Dr. Ahammed Hashim fretted that only 36 percent of the population was immunized and the pace seemed to have stalled. And yet only one or two people in 10 in the local shops wore masks.

“I think the C.D.C. might send a wrong message saying that everything’s OK,” said Dr. Hashim, a pulmonologist. “It would feel much better if we had a 60 or 70 percent vaccination.”

The C.D.C.’s guidance is intended for fully vaccinated individuals, and should only be interpreted as such, Dr. Sharfstein cautioned. Nationwide, only 36 percent of the population is fully vaccinated.

“What we’re just seeing is a little bit of the distance between advice that is entirely appropriate for people who are vaccinated, and the reality that there are places that still are seeing viral transmission and a lot of people who aren’t vaccinated,” he said.

Individuals may make choices based on their perception of their own risks, but state and local leaders must decide what’s best for the community based on the rate of infections. “Those are two different things,” Dr. Sharfstein said. “And when they get conflated, that’s when people may make bad judgments about policy.”

The new guidelines should serve as a reminder to health officials to step up their outreach and investment to ensure that everyone has access to vaccines, Dr. McLaren said. Parents of children under 12 should continue to urge them to wear masks indoors.

The C.D.C.’s new policy shifts the onus onto the immunocompromised as well, to protect themselves from unmasked and unvaccinated people.

“When we make policy, we need to balance the needs and desires of everyone,” Dr. McLaren said. “We could keep masking forever, but there are benefits to getting back to a life that looks more normal.”

Health officials should emphasize that the situation may yet change, and official recommendations with it, she added: “We really need to practice being good at responding to changing situations.”

Covid Vaccines Protect Pregnant Women, Study Confirms

The shots may also have benefits for infants and do not seem to damage the placenta, according to the latest research.

When the coronavirus vaccines were first authorized in December, scientists knew little about how well they might work in pregnant women, who had been excluded from the clinical trials.

Since then, scientists have accumulated a small but steadily growing body of evidence that the vaccines are safe and effective during pregnancy. Preliminary results from two continuing studies provide additional encouraging news.

The Pfizer-BioNTech and Moderna vaccines produce robust immune responses in pregnant and lactating women, and are likely to provide at least some protection against two dangerous coronavirus variants, according to a study published in JAMA on Thursday. Vaccinated women can also pass protective antibodies to their fetuses through the bloodstream and to their infants through breast milk, the research suggests.

In a second study, published in the journal Obstetrics & Gynecology on Tuesday, researchers found no evidence that either the Pfizer or Moderna vaccines damaged the placenta during pregnancy.

“We can shift our framework from, ‘Let’s protect pregnant people from the vaccine,’ to ‘Let’s protect pregnant people and their infants through the vaccine,’” said Dr. Emily S. Miller, an expert in maternal-fetal medicine at Northwestern University and co-author of the placenta study. “I think that’s really powerful.”

Covid presents serious risks during pregnancy. Research has shown, for instance, that pregnant women with coronavirus symptoms are more likely to be admitted to the intensive care unit, require mechanical ventilation and to die from the virus than are symptomatic women of a similar age who are not pregnant.

Because of these risks, the Centers for Disease Control and Prevention has recommended that the vaccines at least be made available to pregnant people, many of whom have opted to receive the shots.

In the JAMA study, scientists at the Beth Israel Deaconess Medical Center in Boston and Harvard Medical School studied blood samples from 103 women who had received the Pfizer or Moderna vaccine between December 2020 and March 2021. Of these women, 30 had received the vaccine while pregnant, 16 while lactating and 57 while neither pregnant nor lactating.

The researchers analyzed the blood samples for signs that the shots had conferred some protection against the coronavirus. Immune responses are complex, and may involve both antibodies — proteins that can bind to and block the virus — and T cells, which help the body recognize the virus and destroy infected cells.

The vaccines produced similar responses in all three groups of women, eliciting both antibody and T-cell responses against the coronavirus, the scientists found. Of particular note, experts said, was the fact that the shots produced high levels of neutralizing antibodies, which can prevent the virus from entering cells, in both pregnant and nonpregnant women.

“Clearly, the vaccines were working in these people,” said Akiko Iwasaki, an immunologist at Yale University who was not involved in the research. “These levels are expected to be quite protective.”

The researchers also found neutralizing antibodies in the breast milk of vaccinated mothers and in umbilical cord blood collected from infants at delivery. “Vaccination of pregnant people and lactating people actually leads to transfer of some immunity to their newborns and lactating infants,” said Dr. Ai-ris Y. Collier, a physician-scientist at Beth Israel who is the first author of the paper.

The results are “really encouraging,” Dr. Iwasaki said. “There is this added benefit of conferring protective antibodies to the newborn and the fetus, which is all the more reason to get vaccinated.”

The scientists also measured the women’s immune responses to two variants of concern: B.1.1.7, which was first identified in Britain, and B.1.351, which was first identified in South Africa. All three groups of women produced antibody and T-cell responses to both variants after vaccination, although their antibody responses were weaker against the variants, especially B.1.351, than against the original strain of the virus, according to the study.

“These women developed immune responses to the variants, although the asterisk is that the antibody responses were reduced several-fold,” said Dr. Dan Barouch, a study author and virologist at Beth Israel. (Dr. Barouch and his colleagues developed the Johnson & Johnson vaccine, which was not included in this study.)

“Overall, it’s good news,” he added. “And it increases the data that suggests that there is a substantial benefit for pregnant women to be vaccinated.”

The researchers also found that 14 percent of pregnant women reported a fever after their second vaccine dose, compared to 52 percent of nonpregnant women. They did not observe any severe complications or side effects.

The study will continue, with researchers monitoring women’s longer-term immune responses. And larger epidemiological studies are still needed to confirm these lab-based results, Dr. Collier noted.

In the second study, a research team from Northwestern University and the Ann and Robert H. Lurie Children’s Hospital of Chicago examined the placentas from 200 women who gave birth between April 2020 and April 2021. Eighty-four of the women had received either the Pfizer or Moderna vaccine during pregnancy; the remainder had not received any coronavirus vaccine.

The placentas from vaccinated women were not any more likely to show signs of injury or abnormality than those from unvaccinated women, the researchers found.

“These data build upon the emerging data that’s come out about these vaccines and their safety in pregnant people,” Dr. Miller said. These are translational data that suggests the placenta doesn’t see any injurious impact of the vaccine. And that’s really fantastic.”

The findings have limitations, she acknowledged. Because the vaccines were only authorized recently, most of the women in the study were vaccinated in the third trimester of pregnancy, and many of them were health care workers, who were among the first people eligible for the shots.

Dr. Miller and her colleagues are continuing to collect more data, including from patients who were vaccinated earlier in their pregnancies and who received the one-shot Johnson & Johnson vaccine.

“This is ongoing work,” Dr. Miller said. But they wanted to publish their preliminary data as soon as they had it, she added, “to help people make the best decision they can.”

C.D.C. Advisers Endorse Pfizer Vaccine for Children Ages 12 to 15

Immunizations will quickly begin nationwide, officials predicted.

The federal government on Wednesday took a final step toward making the Pfizer-BioNTech coronavirus vaccine available to adolescents in the United States, removing an obstacle to school reopenings and cheering millions of families weary of pandemic restrictions.

An advisory committee to the Centers for Disease Control and Prevention voted to recommend the vaccine for use in children ages 12 to 15. The C.D.C. director, Dr. Rochelle Walensky, is expected to review the recommendations and approve them later on Wednesday.

“Approving Covid-19 vaccines for children 12 to 15 years of age is an important step in removing barriers for vaccinating children of all ages,” said Dr. Yvonne Maldonado, who represents the American Academy of Pediatrics on the federal Advisory Committee on Immunization Practices.

Many parents are eagerly anticipating the availability of vaccines for children, at least in part to speed their return to schools. Roughly one-third of eighth graders, usually 13 or 14 years old, are still learning fully remotely.

Vaccinations of adolescents have already begun in a few states, like Maine. Others plan to offer the vaccine as early as Thursday. There are nearly 17 million 12- to 15-year-olds in the United States, accounting for 5.3 percent of the population.

Nearly all states now have a glut of vaccine doses that could be quickly redirected to adolescents. The dose used to immunize adults is also safe and effective for these adolescents, clinical trials have shown.

“Sometimes we lose the importance of children and adolescents in the midst of pandemic — there’s such a focus on older adults in particular,” said Dr. Grace Lee, a member of the committee and professor of pediatrics at Stanford University.

While children’s risk of severe illness is low compared with that of adults, the coronavirus has infected more than 1.5 million children and sent more than 13,000 to hospitals, more than are hospitalized for flu in an average year, according to data collected by the C.D.C.

“It is currently one of the top 10 causes of death in children since the pandemic began,” Dr. Maldonado noted.

Young children are thought to spread the virus less often than adults do, but their ability to transmit increases with age. Teenagers, particularly those in high school, may transmit the virus as readily as adults. Children aged 12 to 17 years represent an increasing proportion of Covid cases in the country.

Vaccinating children should increase the level of immunity in the U.S. population, helping to bring down the number of cases.

“Every person with Covid-19 provides the virus with an opportunity to spread and continue to mutate and further expose our communities,” said Dr. Bill Gruber, a senior vice president at Pfizer. “The decisions from health authorities this week bring us one step closer to protecting adolescents and achieving herd protection.”

Pfizer announced in March that the vaccine seemed to be at least as effective in 12- to 15-year-olds as it has been in older teenagers and adults. Apart from a slight increase in the frequency of fevers, the shots also seemed to have comparable, mostly negligible side effects.

The company plans to continue monitoring trial participants for two years after the second dose to assess the vaccine’s long-term safety and efficacy.

The Food and Drug Administration reviewed the clinical data and on Monday authorized the Pfizer vaccine for use in these children, capping weeks of anticipation from parents and children about a swifter return to normalcy.

“While it’s true that children are generally spared from severe disease, the fact that they’ve been unable to be vaccinated has caused major disruptions in their lives that have real developmental consequences,” said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. “Vaccination of this age cohort will allow these children to more fully return to their normal lives.”

President Biden promised last week to be “ready to move immediately” after authorization of the vaccine for 12- to 15-year-olds. Pediatricians, family doctors, schools and about 20,000 pharmacies nationwide are expected to offer the vaccine for free to these children.

Dr. Anthony S. Fauci, left, and Dr. Rochelle Walensky, the C.D.C. director, before a Senate committee hearing on Tuesday. 
Dr. Anthony S. Fauci, left, and Dr. Rochelle Walensky, the C.D.C. director, before a Senate committee hearing on Tuesday. Pool photo by Jim Lo Scalzo

But some experts have expressed ethical concerns about vaccinating children who are at low risk from the virus, even as health care workers and older adults in many countries remain in danger.

“Thinking purely as a parent, if I had teenagers, I’d probably be happy to vaccinate my children,” said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security.

But, she added, “I greatly worry about a situation where the few countries in the world that have had enough vaccines to protect their adults now continue to hoard those vaccines to use in low-risk kids.”

School reopenings have sputtered across the nation, as parents, teachers, unions and school boards have worried about outbreaks. Research shows that children are mostly spared severe disease and are not significant drivers of coronavirus spread, as they are for influenza.

“Clearly this flawed risk perception will divert vaccination priorities away from what would be the optimal strategic use of the vaccines globally,” he said.

The committee also debated administering the Covid-19 vaccine along with other important vaccines that teenagers may have missed over the year, but deferred a vote to a later date. The agency’s current guidelines suggest waiting two weeks before and after being immunized against Covid-19 before receiving other vaccines.

But the most important hurdle may be parental hesitancy. More than 40 percent of parents of adolescents said they would not get their children vaccinated or would do so only if required by a school, according to a recent survey by the Kaiser Family Foundation.

Some of those parents may change their minds, as other children safely receive vaccines and resume in-person schooling, or rejoin team sports like football and basketball that involve close contact, the researchers suggested.

Others may wait until they must comply with school requirements. Public schools in all 50 states require certain vaccines, but officials may not be able to enforce compliance until the Pfizer-BioNTech vaccine gains the F.D.A.’s full approval.

The vaccine has emergency authorization now. Pfizer has applied to the F.D.A. for full approval, but that process is expected to take several months. Even after approval, students may still opt out by citing medical reasons or religious beliefs.

State and local leaders will need to make particular efforts to reach children in low-income families or in communities of color. Black and Hispanic adults have among the lowest rates of vaccination: As of May 3, just 25 percent of Black people and 27 percent of Hispanic people had been inoculated, compared with 39 percent of white people.

Making the vaccine accessible to these communities will require easier transportation and storage of doses. The Pfizer-BioNTech vaccine can be stored for only five days in standard refrigerators. The companies are planning to ship smaller packs for use in doctors’ offices, and are developing a formulation that can be refrigerated for up to 10 weeks.

Pfizer and BioNTech plan in September to submit requests for authorization of the vaccine in children ages 2 to 11.

To Vaccinate Younger Teens, States and Cities Look to Schools, Camps, Even Beaches

The F.D.A.’s authorization of Pfizer’s Covid shot for 12- to 15-year-olds is a milestone in battling the coronavirus, but actually getting them vaccinated involves new challenges.

Abby Goodnough and

Hundreds of high school seniors rode in bus caravans recently to a mass vaccination site outside Hartford, Conn., where they got Covid-19 shots as a D.J. played Lady Gaga and a selfie backdrop awaited.

Now, the race is on to get to their younger siblings — and all the nation’s nearly 17 million 12- to 15-year-olds, after the Food and Drug Administration authorized the Pfizer-BioNTech vaccine for their age group.

The F.D.A.’s decision, announced Monday afternoon, presents a bright new opportunity in the push for broad immunity against the coronavirus in the United States, but the challenges are more daunting than for immunizing older, more independent teenagers.

A recent survey by the Kaiser Family Foundation’s Vaccine Monitor found that many parents — even those who eagerly got their own Covid shots — are reluctant to vaccinate pubescent children. Yet doing so will be critical for further reducing transmission of the virus, smoothly reopening middle and high schools and regaining some sense of national normalcy.

“The game changes when you go down as young as 12 years old,” said Nathan Quesnel, the superintendent of schools in East Hartford, adding, “You need to have a different level of sensitivity.”

States, counties and school districts around the country are trying to figure out the most reassuring and expedient ways to reach younger adolescents as well as their parents, whose consent is usually required by state law. They are making plans to offer vaccines not only in schools, but also at pediatricians’ offices, day camps, parks and even beaches.

Children’s Minnesota, a Minneapolis-based hospital system where the main Covid vaccination site has offered stress balls, colored lights and images of playful dolphins projected on the ceiling, is planning to provide shots beginning later this week in at least a dozen middle schools and a Y.W.C.A.

In Columbus, Ohio, public health nurses will drive a mobile vaccination unit around neighborhoods “just like you would an ice cream truck,” said Dr. Mysheika Roberts, the city health commissioner. In Connecticut, Community Health Center, a statewide primary care provider that vaccinated the busloads of high school seniors, is aiming to reach younger adolescents by offering shots at amusement parks, beaches and camps, among other locales.

“You’re going to Dollar General?” said Yvette Highsmith-Francis, a vice president of Community Health Center. “Guess what? We’re in the parking lot.”

But with the school year ending soon, many health officials are racing against the academic clock to schedule both recommended doses, seeing schools as the best place to reach many students at once.

A vial of the Pfizer vaccine at the Denver School of Science and Technology Green Valley Ranch this month.
A vial of the Pfizer vaccine at the Denver School of Science and Technology Green Valley Ranch this month.Kevin Mohatt for The New York Times

“We have a very finite amount of time,” said Dr. Anne Zink, the chief medical officer for Alaska. “In Alaska, kids go to the wind as soon as summer hits, so our opportunity to get them is now.”

A number of places are revving up vaccination efforts in schools. In Colorado, Denver Health will expand clinics it operates in six public schools to middle school students. For the last few weeks, it has provided 150 to 400 vaccines every Saturday and Sunday, reaching not just high school juniors and seniors but sometimes their parents and older siblings, too.

“It’s been really successful because we are doing it in their communities, where the kids are familiar,” said Dr. Sonja O’Leary, the medical director for Denver Health’s school-based health centers.

Other states believe pediatricians’ and family doctors’ offices will be the best places to catch teenagers — and children as young as infants as companies plan eventually to seek authorization for the shots to be given to the youngest children. Until recently, few doctors had vaccines on hand for patients. But in recent weeks, the Centers for Disease Control and Prevention has made a major push to enroll pediatricians to give the shots.

The thinking is that pediatricians are in the best position to field questions from parents and children. Not only are they experienced in giving routine childhood vaccinations, but they are also often a household’s most trusted source of health information.

President Biden announced plans last week to ship doses of the Pfizer vaccine directly to pediatricians’ offices, and he said that about 20,000 pharmacy sites were also ready to administer the vaccine to younger adolescents.

There are also practical issues. Staggering Covid shots around the routine vaccines required for school in September — which many children are behind on because of the pandemic — will be complicated. According to the C.D.C., no vaccines can be given two weeks before and after a Covid vaccine.

Pediatricians are used to talking to nervous parents about vaccines, but they concede that the Covid shot poses unique persuasion challenges. To help these conversations, the American Academy of Pediatrics has posted answers to frequently asked questions and has been holding virtual training workshops.

Pediatricians say they have been getting vaccine questions for months.

From left, Garrett Bates, his sons Christopher Bates and Preston Janac; Precious Wright, his partner; and his daughter Tyshawnie Bates, of Hollywood, Fla.Octavio Jones for The New York Times

Many parents and teenagers have been stirred by false information coursing across the internet about the shots’ impact on fertility and menstrual cycles, said Dr. Hina Talib, an adolescent medicine specialist at Children’s Hospital at Montefiore Medical Center in the Bronx, who posts on Instagram as @teenhealthdoc.

“With hormones floating around during puberty, parents ask if it’s dangerous for their child to be given a vaccine during that time,” Dr. Talib said. The questions reflect the parents’ thoughtfulness, she said, and need to be addressed respectfully.

Dr. Talib, whose patients are often Black or Latino and recent immigrants, said that many hear vaccine resistance at home. “We have to validate parental anxiety and mistrust of medicine and be very open to listening to what their experiences have been,” she said.

Garrett Bates and Precious Wright, who live in Hollywood, Fla., have tentatively decided to get themselves vaccinated, but they are holding off on their four children, ages 12 through 19, just now.

It has been a tough year: Two of the children attended school in person, two were remote. Yet, even though vaccination offers the possibility that all their children will have a more engaged, carefree life, Ms. Wright wants to see how others their age fare first.

“From what I know, you take the vaccine and some people feel sick and it lasts a couple of hours or a day,” she said. “My immune system is stronger than the kids’. I don’t know if they could shake off those effects as quickly as mine.”

For some teenagers, anxious about bringing the virus home to vulnerable relatives, the vaccine represents liberation — from those worries as well as constraints on seeing friends.

“The kids have ‘shot envy,’” Dr. Talib said.

Dr. Nicole Baldwin, a pediatrician in Cincinnati whose health-related TikTok videos now feature one for the Pfizer vaccine, said she was surprised by how excited many of her teenaged patients were about the vaccine. “I’ll ask, ‘Have your friends gotten it?’ And they’re saying, ‘Yes!’”

But she also has patients, including those with high-risk medical conditions that make them vulnerable to Covid, who are not getting it. “Their parents say no,” she said.

When parent and child are at odds about the vaccine, the pediatrician has a tricky path to walk. And when divorced parents disagree over whether their child should get the vaccine, those discussions become even more difficult.

Not all teenagers long for the vaccine. Many hate getting shots. Others say that because young people often get milder cases of Covid, why risk a new vaccine?

Patsy Stinchfield, a nurse practitioner who oversees vaccination for Children’s Minnesota, has stark evidence that some cases in young people can be serious. Not only have more children with Covid been admitted to the hospital recently, but its intensive care unit also has Covid patients who are 13, 15, 16 and 17 years old.

The F.D.A.’s new authorization means all those patients would be eligible for the shots, she noted. “If you can prevent your child ending up in the I.C.U. with a safe vaccine, why wouldn’t you ?” she said.

Mr. Quesnel, the East Hartford, Conn., superintendent, said the most powerful message for reaching older adolescents would probably appeal just as much to younger ones. Rather than focusing on the fact that the shot will protect them, he said, they seize on the idea that it will keep them from having to quarantine if they are exposed.

“They’re not so afraid of the health care dangers from Covid but the social losses that come along with it,” he said, adding that 60 percent of his district’s seniors, or about 300 students, got their first dose at a mass vaccination site run by Community Health Center on April 26. “Some of our greatest leverage right now is that social component — ‘You won’t be quarantined.’”

Michael Jackson of North Port, Fla., can’t wait for his 14-year-old son, Devin, to get the vaccine. During the past year, he said, his son’s beloved Little League games went on hiatus and the family had to suspend their regular Sunday suppers with grandparents And Devin, an eighth grader, had to quarantine three times after being exposed to Covid.

Other parents have challenged Mr. Jackson about his plans to get Devin vaccinated. “They say to me, ‘How can you put that in your body?’” he said, adding, “And meanwhile they’re eating a Big Mac and drinking a can of soda?”

Michael Jackson, left, of North Port, Fla., is eager for his 14-year-old son, Devin, to get vaccinated.Octavio Jones for The New York Times

Before any younger adolescents can receive the shots, the C.D.C.’s vaccine advisory committee will meet in the coming days to review the clinical trial data and make recommendations for the vaccine’s use in the 12-to-15 age group.

Within months, eligibility for the vaccines is expected to expand to even younger children. Pfizer expects to seek emergency authorization in September to administer its vaccine to children between the ages of 2 and 11. Moderna’s clinical trial results for its vaccine in 12- to 17-year-olds are expected in the next few weeks, and those from a trial of its vaccine in children 6 months to 12 years old in the second half of this year.

All 50 states require certain vaccines for children who attend school, but those mandates apply only to vaccines that have been fully approved by the F.D.A., a status the Covid shots have not yet achieved. And even when the F.D.A. approves the vaccines, any state-legislated mandates would most likely allow students to opt out for medical, religious and sometimes even philosophical reasons, as they do for other childhood shots.

In Columbus, Dr. Roberts already has a good sense of the challenge ahead. Her department worked with the local children’s hospital to offer vaccinations to older teenagers at high schools over the last month, hoping to reach up to 6,000 of them.

“We only got about 600,” she said, noting that parental fears about infertility were the most common reason for refusing the shots. Now she and her staff are considering offering incentives like free meals and grocery store gift cards to parents, and perhaps prizes for children as well.

“We’re committed to getting this population vaccinated,” she said, “so we’re going to look at anything and everything.”

FDA Authorizes Pfizer-BioNTech Vaccine for Children 12 to 15

The shots may allow millions of youngsters to get back to school, camps, sleepovers and hangouts with friends.

The Food and Drug Administration on Monday authorized use of the Pfizer-BioNTech Covid-19 vaccine for 12- to 15-year-olds in the United States, a crucial step in the nation’s steady recovery from the pandemic and a boon to millions of American families eager for a return to normalcy.

The authorization caps weeks of anticipation among parents, who have been grappling with how to conduct their lives when only the adults in a household are immunized. It removes an obstacle to school reopenings by reducing the threat of transmission in classrooms, and affords more of the nearly 17 million children in this age group opportunities to attend summer camps, sleepovers and Little League games.

“This is great news,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “It feels like we’ve been waiting a long time to start protecting children in this age group.” The Pfizer-BioNTech vaccine is already available to anyone over 16.

The F.D.A.’s go-ahead is not the final hurdle. An advisory committee of the Centers for Disease Control and Prevention is expected to meet shortly to review the data and make recommendations for the vaccine’s use in 12- to 15-year-olds.

If the committee endorses the vaccine for that age group, as expected, immunizations in theory can begin immediately. Clinical trials have shown that these children may safely receive the dose already available for adults.

President Biden has said that about 20,000 pharmacies are ready to administer the vaccine to adolescents, and health officials in some states are already drawing up immunization campaigns targeted to youngsters.

In a clinical trial, Pfizer and BioNTech enrolled 2,260 participants ages 12 and 15 and gave them either two doses of the vaccine or a placebo three weeks apart. The researchers recorded 18 cases of symptomatic coronavirus infection in the placebo group, and none among the children who received the vaccine, indicating that it was highly effective at preventing symptomatic illness.

The vaccine also appeared to be safe for these children, with side effects comparable to those seen in trial participants who are 16 to 25 years old. Fevers were slightly more common among inoculated 12- to 15-year-olds; about 20 percent of them had fevers, compared with 17 percent in the older age group.

The trend toward more fevers at younger ages was consistent with observations in an earlier trial, said Dr. Bill Gruber, a senior vice president at Pfizer and a pediatrician.

The trial results were a “trifecta” of good news, Dr. Gruber added: “We have safety, we got the immune response we wanted — it was actually better than what we saw in the 16- to 25-year-old population — and we had outright demonstration of efficacy.”

The company is still gathering information on potential asymptomatic infections by continuing to test the trial participants for the coronavirus every two weeks and checking them for antibodies produced in response to a natural infection, according to Dr. Gruber.

The authorization arrives at a opportune time. Roughly one-third of eighth graders, usually 13 and 14 years old, are still learning fully remotely, and widespread vaccination may help speed a return to classrooms.

Coronavirus outbreaks have been a concern particularly for students participating in team sports, and immunizations should ease the concerns of many parents whose children have been unable to participate in football, basketball and other team sports involving close contact.

In summer, families often catch up the vaccinations required for children to return to schools in the fall, and so pediatricians and family doctors may be best positioned to immunize young teenagers as they come in for annual physical examinations.

The Pfizer-BioNTech vaccine can be stored for only five days in standard refrigerators, but the companies are planning to ship smaller packs for doctors’ offices. It is also developing a formulation that can be refrigerated for up to 10 weeks.

The push to immunize children may run into the same problems with hesitancy that have plagued attempts to inoculate adults. In one recent poll, 30 percent of parents said they would have their children vaccinated right away, while 26 percent said they planned to wait to see how the vaccine was working.

Most of the other parents said they would definitely not have their children vaccinated, or would do so only if schools required it. All 50 states require certain vaccines for children who attend school, but those mandates apply only to vaccines that have been fully approved by the F.D.A. The Pfizer-BioNTech vaccine has been authorized only for emergency use.

The companies have applied to the F.D.A. for full approval, but that process is expected to take several months. Even after approval, students may still opt out by citing medical reasons or religious beliefs.

Still, scientists agreed that the vaccine appeared to meet all expectations regarding safety and efficacy. Dr. Megan Ranney, an emergency room physician and professor at Brown University in Providence, R.I., said she had “zero safety concerns” about the Pfizer-BioNTech vaccine, noting that hundreds of millions of people worldwide had received it.

Her 12-year-old daughter is eager to be vaccinated, and her 9-year-old son will be immunized as soon as he is eligible, she said. Dr. Ranney has not allowed her children to sleep at friends’ houses since the pandemic began. The vaccine should allow them to safely resume social activities, she said.

“The risk of your child catching Covid and getting really sick is low, but it’s not zero,” she said. “And the risk of them getting sick or hospitalized or worse with Covid or with the post-Covid multi-inflammatory syndrome is higher than the risk of something bad from this vaccine.”

Vaccinating children shields others in the community from the virus, she noted, including people who are not protected by the vaccine, such as organ transplant recipients, cancer patients and those with impaired immune responses.

“It also protects all of us from the virus continuing to spread and mutating further,” Dr. Ranney said. “That’s the thing that I’m most scared of right now.”

Vaccinating children is crucial to building up population levels of immunity and curtailing the spread of the coronavirus. Though children spread the virus less efficiently than adults do, they make up about 23 percent of the population.

Experts have said that the country is unlikely to reach the “herd immunity” threshold — the point at which virus transmission essentially stalls — but vaccinating children will be important for getting as close as possible.

Ty Dropic, 14, one of the trial participants, urged others his age to be vaccinated so they could build up widespread immunity and protect themselves. He had no side effects, leading him to suspect that he got the placebo. If that turns out to be the case, he plans to be immunized as soon as possible.

“I know it can be kind of scary, but it’s really not as bad as it seems,” he said. “If you do get Covid, it’ll be a lot worse than getting stuck with a needle for, like, two seconds.”

Ty’s three siblings, ages 8, 10 and 16, are also enrolled in vaccine trials for their age groups. Their mother, Dr. Amanda Dropic, a pediatrician in northern Kentucky, said that in her practice, most parents were eager to have their children vaccinated so they could regain some semblance of normalcy.

“The anxiety and depression that we’re seeing with kids, the social delays, has been tremendous,” she said.

Dr. Dropic said her children understood the risks and were willing to volunteer because they saw it as a civic duty. Every medicine available today came to be because “somebody was willing to go first,” she added.

Pfizer and BioNTech began testing the vaccine in children ages 5 to 11 in March, and last month extended the trial to even younger children, ages 2 to 5. The companies next plan to test children who are 6 months to 2 years old.

Assuming trial results are encouraging, the companies expect to apply to the F.D.A. in September for emergency authorization to administer the vaccine to children ages 2 to 11.

Results from trials of Moderna’s vaccine in 12- to 17-year-olds are expected in the next few weeks. Findings from another trial of the company’s vaccine in children 6 months to 12 years old should be available in the second half of this year.

AstraZeneca is testing its vaccine in children 6 months and older. Johnson & Johnson plans to wait for results from trials in participants older than 12 before testing its vaccine in younger children.

Jan Hoffman contributed reporting.

F.D.A. Authorizes Pfizer-BioNTech Vaccine for Children 12 to 15

The shots may allow millions of youngsters to get back to school, camps, sleepovers and hangouts with friends.

The Food and Drug Administration on Monday authorized use of the Pfizer-BioNTech Covid-19 vaccine for 12- to 15-year-olds in the United States, a crucial step in the nation’s steady recovery from the pandemic and a boon to millions of American families eager for a return to normalcy.

The authorization caps weeks of anticipation among parents, who have been grappling with how to conduct their lives when only the adults in a household are immunized. It removes an obstacle to school reopenings by reducing the threat of transmission in classrooms, and affords the nearly 17 million children in this age group the opportunity to attend summer camps, sleepovers and Little League games.

“This is great news,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “It feels like we’ve been waiting a long time to start protecting children in this age group.” The Pfizer-BioNTech vaccine is already available to anyone over 16.

The F.D.A.’s go-ahead is not the final hurdle. An advisory committee of the Centers for Disease Control and Prevention is expected to meet shortly to review the data and make recommendations for the vaccine’s use in 12- to 15-year-olds.

If the committee endorses the vaccine for that age group, as expected, immunizations in theory can begin immediately. Clinical trials have shown that these children may safely receive the dose already available for adults.

President Biden has said that about 20,000 pharmacies are ready to administer the vaccine to adolescents, and health officials in some states are already drawing up immunization campaigns targeted to youngsters.

In a clinical trial, Pfizer and BioNTech enrolled 2,260 participants ages 12 and 15 and gave them either two doses of the vaccine or a placebo three weeks apart. The researchers recorded 18 cases of symptomatic coronavirus infection in the placebo group, and none among the children who received the vaccine, indicating that it was highly effective at preventing symptomatic illness.

The vaccine also appeared to be safe for these children, with side effects comparable to those seen in trial participants who are 16 to 25 years old. Fevers were slightly more common among inoculated 12- to 15-year-olds; about 20 percent of them had fevers, compared with 17 percent in the older age group.

The trend toward more fevers at younger ages was consistent with observations in an earlier trial, said Dr. Bill Gruber, a senior vice president at Pfizer and a pediatrician.

The trial results were a “trifecta” of good news, Dr. Gruber added: “We have safety, we got the immune response we wanted — it was actually better than what we saw in the 16- to 25-year-old population — and we had outright demonstration of efficacy.”

The company is still gathering information on potential asymptomatic infections by continuing to test the trial participants for the coronavirus every two weeks and checking them for antibodies produced in response to a natural infection, according to Dr. Gruber.

The authorization arrives at a opportune time. Roughly one-third of eighth graders, usually 13 and 14 years old, are still learning fully remotely, and widespread vaccination may help speed a return to classrooms.

Coronavirus outbreaks have been a concern particularly for students participating in team sports, and immunizations should ease the concerns of many parents whose children have been unable to participate in football, basketball and other team sports involving close contact.

In summer, families often catch up the vaccinations required for children to return to schools in the fall, and so pediatricians and family doctors may be best positioned to immunize young teenagers as they come in for annual physical examinations.

The Pfizer-BioNTech vaccine can be stored for only five days in standard refrigerators, but the companies are planning to ship smaller packs for doctors’ offices. It is also developing a formulation that can be refrigerated for up to 10 weeks.

The push to immunize children may run into the same problems with hesitancy that have plagued attempts to inoculate adults. In one recent poll, 30 percent of parents said they would have their children vaccinated right away, while 26 percent said they planned to wait to see how the vaccine was working.

Most of the other parents said they would definitely not have their children vaccinated, or would do so only if schools required it. All 50 states require certain vaccines for children who attend school, but those mandates apply only to vaccines that have been fully approved by the F.D.A. The Pfizer-BioNTech vaccine has been authorized only for emergency use.

The companies have applied to the F.D.A. for full approval, but that process is expected to take several months. Even after approval, students may still opt out by citing medical reasons or religious beliefs.

Still, scientists agreed that the vaccine appeared to meet all expectations regarding safety and efficacy. Dr. Megan Ranney, an emergency room physician at Rhode Island Hospital in Providence, said she had “zero safety concerns” about the Pfizer-BioNTech vaccine, noting that hundreds of millions of people worldwide had received it.

Her 12-year-old daughter is eager to be vaccinated, and her 9-year-old son will be immunized as soon as he is eligible, she said. Dr. Ranney has not allowed her children to sleep at friends’ houses since the pandemic began. The vaccine should allow them to safely resume social activities, she said.

“The risk of your child catching Covid and getting really sick is low, but it’s not zero,” she said. “And the risk of them getting sick or hospitalized or worse with Covid or with the post-Covid multi-inflammatory syndrome is higher than the risk of something bad from this vaccine.”

Vaccinating children shields others in the community from the virus, she noted, including people who are not protected by the vaccine, such as organ transplant recipients, cancer patients and those with impaired immune responses.

“It also protects all of us from the virus continuing to spread and mutating further,” Dr. Ranney said. “That’s the thing that I’m most scared of right now.”

Vaccinating children is crucial to building up population levels of immunity and curtailing the spread of the coronavirus. Though children spread the virus less efficiently than adults do, they make up about 23 percent of the population.

Experts have said that the country is unlikely to reach the “herd immunity” threshold — the point at which virus transmission essentially stalls — but vaccinating children will be important for getting as close as possible.

Ty Dropic, 14, one of the trial participants, urged others his age to be vaccinated so they could build up widespread immunity and protect themselves. He had no side effects, leading him to suspect that he got the placebo. If that turns out to be the case, he plans to be immunized as soon as possible.

“I know it can be kind of scary, but it’s really not as bad as it seems,” he said. “If you do get Covid, it’ll be a lot worse than getting stuck with a needle for, like, two seconds.”

Ty’s three siblings, ages 8, 10 and 16, are also enrolled in vaccine trials for their age groups. Their mother, Dr. Amanda Dropic, a pediatrician in northern Kentucky, said that in her practice, most parents were eager to have their children vaccinated so they could regain some semblance of normalcy.

“The anxiety and depression that we’re seeing with kids, the social delays, has been tremendous,” she said.

Dr. Dropic said her children understood the risks and were willing to volunteer because they saw it as a civic duty. Every medicine available today is because “somebody was willing to go first,” she added.

Pfizer and BioNTech began testing the vaccine in children ages 5 to 11 in March, and last month extended the trial to even younger children, ages 2 to 5. The companies next plan to test children who are 6 months to 2 years old.

Assuming trial results are encouraging, the companies expect to apply to the F.D.A. in September for emergency authorization to administer the vaccine to children ages 2 to 11.

Results from trials of Moderna’s vaccine in 12- to 17-year-olds are expected in the next few weeks. Findings from another trial of the company’s vaccine in children 6 months to 12 years old should be available in the second half of this year.

AstraZeneca is testing its vaccine in children 6 months and older. Johnson & Johnson plans to wait for results from trials in participants older than 12 before testing its vaccine in younger children.

Jan Hoffman contributed reporting.

Pfizer-BioNTech Vaccine is Highly Effective Against Variants, Studies Find

Two studies showed the vaccine to be more than 95 percent effective at protecting against severe disease or death from the variants first identified in South Africa and the U.K.

The Pfizer-BioNTech coronavirus vaccine is extraordinarily effective at protecting against severe disease caused by two dangerous variants, according to two studies published Wednesday.

The studies, which are based on the real-world use of the vaccine in Qatar and Israel, suggest that the vaccine can prevent the worst outcomes — including severe pneumonia and death — caused by B.1.1.7, the variant first identified in the U.K., and B.1.351, the variant first identified in South Africa.

“This is really good news,” said Dr. Annelies Wilder-Smith, an infectious disease researcher at the London School of Hygiene and Tropical Medicine. “At this point in time we can confidently say that we can use this vaccine, even in the presence of circulating variants of concern.”

Previous research suggested that B.1.1.7 is more infectious and more deadly than other variants, but that vaccines still worked well against it. But vaccines appeared to be less effective against B.1.351, according to earlier studies.

One of the new studies, which appeared in the New England Journal of Medicine, is based on information about more than 200,000 people that was pulled from Qatar’s national Covid-19 databases between Feb. 1 and March 31.

During that time, the variants were widespread there: Sequencing conducted between Feb. 23 and March 18 suggested that roughly half of the coronavirus infections in that period were caused by B.1.351 and 44.5 percent were caused by B.1.1.7.

In multiple analyses, the researchers found that the vaccine was 87 to 89.5 percent effective at preventing infection with B.1.1.7 among people who were at least two weeks past their second shot. It was 72.1 to 75 percent effective at preventing infection with B.1.351 among those who had reached the two-week point.

Even that slightly reduced effectiveness against infection with B.1.351 is still largely good news, one of the study’s authors, Laith Abu-Raddad, an infectious disease epidemiologist at Weill Cornell Medicine-Qatar, said. “We’re talking about a variant which is probably the nastiest of all the variants of concern,” he said. “It’s not the 95 percent we were hoping, but the 75 percent is really great.”

The vaccine was highly effective at protecting against the worst outcomes. Over all, it was 97.4 percent effective at preventing severe, critical or fatal disease from any form of the coronavirus, and 100 percent effective at preventing severe, critical or fatal disease caused by B.1.1.7 or B.1.351. (This slight difference in effectiveness is likely a result of the fact that the sample sizes were smaller for the subgroups of patients with a documented variant, Dr. Abu-Raddad said.)

The second new study, which was published in The Lancet, was conducted by researchers at the Israel Ministry of Health and Pfizer. It is based on more than 230,000 coronavirus infections that occurred in Israel between Jan. 24 and April 3. During that period, B.1.1.7 accounted for nearly 95 percent of all coronavirus cases in the country, which has vaccinated more than half of its population.

The researchers found that the vaccine was more than 95 percent effective at protecting against coronavirus infection, hospitalization and death among fully vaccinated people 16 and older. It also worked well in older adults. Among those 85 or older, the vaccine was more than 94 percent effective at protecting against infection, hospitalization and death.

As the percentage of fully vaccinated people in each age group grew, the incidence of coronavirus infections in that cohort fell, the researchers found. The declines in infection rates matched the timing of increasing vaccine coverage in each age group better than the start of a nationwide lockdown. The results suggest that Israel’s rapid pace of vaccination has been responsible for the decline in infections in the country.

I’m just really happy to see this data that in the real world these vaccines are having such an amazing impact on curtailing infection and disease,” said Akiko Iwasaki, an immunologist at Yale University.

Both studies also reported that two doses of the vaccine provided significantly more protection than one dose did. In the Israel study, for example, one dose of the vaccine was 77 percent effective against death, while two doses were 96.7 percent effective.

“It absolutely emphasizes the need for the second dose,” said Dr. Kathleen Neuzil, who directs the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine.

Together, the studies suggest that even with the new variants, vaccination remains a plausible path out of the pandemic, experts said. “If we can get vaccines to the world and get coverage up,” Dr. Neuzil said, “I believe we can get on top of this and we can get on top of the emergence of new variants.”

Vaccines Are Effective Against the New York Variant, Studies Find

The research adds to a growing number of findings suggesting the Pfizer and Moderna shots are protective against the variants identified so far.

For weeks, New Yorkers have witnessed the alarming rise of a homegrown variant of the coronavirus that has kept the number of cases in the city stubbornly high. City officials have repeatedly warned that the variant may be more contagious and may dodge the immune response.

On that second point, at least, they can now breathe easier: Both the Pfizer-BioNTech and Moderna vaccines will effectively prevent serious illness and death from the variant, two independent studies suggest.

Antibodies stimulated by those vaccines are only slightly less potent at controlling the variant than the original form of the virus, both studies found.

“We’re not seeing big differences,” said Michel Nussenzweig, an immunologist at Rockefeller University in New York and a member of the team that published one of the studies on Thursday.

The bottom line? “Get vaccinated,” he said.

The results are based on laboratory experiments with blood samples from small numbers of vaccinated people and have not yet been peer-reviewed. Still, they are consistent with what is known about similar variants, several experts said, and they add to a growing body of research that suggests that the two main vaccines in the United States are protective against all of the variants identified so far.

“The take-home message is that the vaccines are going to work against the New York variant and the South African variant and the U.K. variant,” said Nathan Landau, a virologist at N.Y.U.’s Grossman School of Medicine who led the study.

The vaccines spur the body to mount an expansive immune response, with thousands of types of antibodies and several types of immune cells. One subset of these immune fighters, called neutralizing antibodies, is essential for preventing infection. But even when neutralizing antibodies are in short supply or even absent, the rest of the immune system may marshal enough of a defense to fend off serious illness and death.

In both new studies, neutralizing antibodies from vaccinated people were better at thwarting the virus than those from people who developed antibodies from being sick with Covid-19. Direct comparison of the two sets of antibodies offered a possible explanation: Antibodies from vaccinated people are distributed across a broader range of parts of the virus, so no single mutation has a big impact on their effectiveness — making vaccines a better bet against variants than immunity from natural infection.

The variant first identified in New York, known to scientists as B.1.526, raced through the city after its initial discovery in November. It accounted for one in four diagnosed cases by November and nearly half of cases as of April 13. The variant that brought Britain to a standstill, B.1.1.7, is also circulating widely in New York. Together, the two add up to more than 70 percent of coronavirus cases in the city.

The worry about the variant identified in New York has centered on one form of it, which contains a mutation that scientists are calling Eek. The Eek mutation subtly alters the shape of the virus, making it difficult for antibodies to target the virus and, as a result, undercutting the vaccines.

In the second study, Dr. Landau’s team found that the Pfizer and Moderna vaccines are only marginally less protective against the variant that devastated Britain and against forms of the variant discovered in New York that don’t contain the Eek mutation.

Several laboratory studies have shown that antibodies induced by the Pfizer and Moderna vaccines are slightly less powerful against a third variant, one identified in South Africa, which also contains Eek. Other vaccines fared worse. South Africa suspended use of the AstraZeneca vaccine after clinical trials showed that the vaccine did not prevent mild or moderate illness from the variant that was circulating there.

“It already started out as a lower level in terms of the immunity that it generated,” Dr. Nussenzweig said of the AstraZeneca vaccine. Referring to the Pfizer and Moderna shots, he said, “We’re so lucky in this country to have these vaccines compared to the rest of the world.”

Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai who was not involved in either of the new studies, said he was more concerned about other countries’ vaccine programs than about the variants themselves.

“I am less concerned about variants than I was two months ago,” he said, but added: “I’m worried about countries that don’t have enough vaccine and that don’t have that vaccine rollout. I’m not worried anymore about the U.S., honestly.”

Dr. Landau’s team also tested monoclonal antibodies used to treat Covid-19 against the variants. They found that the cocktail of monoclonal antibodies made by Regeneron worked as well against the variant discovered in New York as against the original virus.

The studies are reassuring, but they indicate that the Eek mutation is one to watch, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

“This could certainly be a step toward the virus becoming somewhat more resistant to infection- and vaccine-mediated immunity,” Dr. Bloom said. “I don’t think it’s something that people need to immediately become alarmed about, but it definitely impresses us as important.”

Dr. Bloom led the analysis comparing vaccine-induced antibodies with those produced by natural infection. He found that the most powerful antibodies bind to multiple sites in a key part of the virus. Even if a mutation affects the binding in one site in this region, antibodies that target the remaining sites would still be protective.

Antibodies induced by the vaccine cover many more sites across this region than those from natural infection — and so are less likely to be affected by a mutation in any one site.

The study looked only at antibodies stimulated by the Moderna vaccine, but the results are likely to be the same for the Pfizer-BioNTech vaccine, he added.

“This could potentially be a good thing as the virus is creating mutations,” Dr. Bloom said.

Can the Covid Vaccine Protect Me Against Virus Variants?

Vaccines do a good job of protecting us from coronavirus, but fear and confusion about the rise of variants have muddled the message. Here are answers to common questions.

The news about coronavirus variants can sound like a horror movie, with references to a “double-mutant” virus, “vaccine-evading” variants and even an “Eek” mutation. One headline warned ominously: “The devil is already here.”

While it’s true that the virus variants are a significant public health concern, the unrelenting focus on each new variant has created undue alarm and a false impression that vaccines don’t protect us against the various variants that continue to emerge.

“I use the term ‘scariants,’” said Dr. Eric Topol, professor of molecular medicine at Scripps Research in La Jolla, Calif., referring to much of the media coverage of the variants. “Even my wife was saying, ‘What about this double mutant?’ It drives me nuts. People are scared unnecessarily. If you’re fully vaccinated, two weeks post dose, you shouldn’t have to worry about variants at all.”

Viruses are constantly changing, and new variants have been emerging and circulating around the world throughout the pandemic. Some mutations don’t matter, but others can make things much worse by creating a variant that spreads faster or makes people sicker. While the rise of more infectious variants has caused cases of Covid-19 to surge around the world, the risk is primarily to the unvaccinated, for whom there is great concern. While vaccination efforts are well underway in the United States and many other developed countries, huge swaths of the world’s population remain vulnerable, with some countries yet to report having administered a single dose.

But for the vaccinated, the outlook is much more hopeful. While it’s true that the vaccines have different success rates against different variants, the perception that they don’t work against variants at all is incorrect. In fact, the available vaccines have worked remarkably well so far, not just at preventing infection but, most important, at preventing serious illness and hospitalization, even as new variants circulate around the globe.

The variants are “all the more reason to get vaccinated,” said Dr. Anthony S. Fauci, the nation’s top infectious disease specialist. “The bottom line is the vaccines we are using very well protect against the most dominant variant we have right now, and to varying degrees protect against serious disease among several of the other variants.”

Part of the confusion stems from what vaccine efficacy really means and the use of terms like “vaccine evasion,” which sounds a lot scarier than it is. In addition, the fact that two vaccines have achieved about 95 percent efficacy has created unrealistic expectations about what it takes for a vaccine to perform well.

Here are answers to common questions about the variants and the vaccines being used to stop Covid-19.

Which variant am I most likely to encounter in the United States?

The variant called B.1.1.7, which was first identified in Britain, is now the most common source of new infections in the United States. This highly contagious variant is also fueling the spread of the virus in Europe and has been found in 114 countries. A mutation allows this version of the virus to more effectively attach to cells. Carriers may also shed much higher levels of virus and stay infectious longer.

The main concern about B.1.1.7 is that it is highly infectious and spreads quickly among the unvaccinated, potentially overwhelming hospitals in areas where cases are surging.

Do the vaccines work against B.1.1.7?

All of the major vaccines in use — Pfizer-BioNTech, Moderna, Johnson & Johnson, AstraZeneca, Sputnik and Novavax — have been shown to be effective against B.1.1.7. We know this from a variety of studies and indicators. First, scientists have used the blood of vaccinated patients to study how well vaccine antibodies bind to a variant in a test tube. The vaccines have all performed relatively well against B.1.1.7. There’s also clinical trial data, particularly from Johnson & Johnson and AstraZeneca (which is the most widely used vaccine around the world), that shows they are highly effective against both preventing infection and serious illness in areas where B.1.1.7 is circulating. And in Israel, for instance, where 80 percent of the eligible population is vaccinated (all with the Pfizer shot), case counts are plummeting, even as schools, restaurants and workplaces open up, suggesting that vaccines are tamping down new infections, including those caused by variants.

If the vaccines are working, why do I keep hearing about “breakthrough” cases?

No vaccine is foolproof, and even though the Covid vaccines are highly protective, sometimes vaccinated people still get infected. But breakthrough cases of vaccinated people are very rare, even as variants are fueling a surge in case counts. And the vaccines clearly prevent severe illness and hospitalization in the few vaccinated patients who do get infected.

So what’s the risk of getting infected after vaccination? Nobody knows for sure, but we have some clues. During the Moderna trial, for instance, only 11 patients out of 15,210 who were vaccinated got infected. Both Pfizer and Moderna now are doing more detailed studies of breakthrough cases among vaccinated trial participants, and should be releasing that data soon.

Two real-world studies of vaccinated health care workers, who have a much higher risk of virus exposure than the rest of us, offer hopeful signs. One study found that just four out of 8,121 fully vaccinated employees at the University of Texas 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 were published in the New England Journal of Medicine, and are a sign that even as cases were surging in the United States, breakthrough cases were uncommon, even among individuals who were often exposed to sick patients. Most important, patients who were infected after vaccination had mild symptoms. Some people had no symptoms at all, and were discovered only through testing in studies or as part of their unrelated medical care.

Researchers are still studying whether the variants eventually might increase the number of breakthrough cases or if vaccine antibodies begin to wane over time. (So far, data from Moderna show the vaccine still remains 90 percent effective after at least six months. Pfizer has reported similar results.

A recent study of 149 people in Israel who became infected after vaccination with the Pfizer vaccine suggested that a variant first identified in South Africa was more likely to cause breakthrough infections. However, those infections occurred between days seven and 13 following the second dose. “We didn’t see any South Africa variant 14 days after the second dose,” said Adi Stern, the study’s senior author, a professor at the Shmunis School of Biomedicine and Cancer Research, Tel Aviv University. “It was a small sample size, but it’s very possible that two weeks after the second dose, maybe the protection level goes up and that blocks the South Africa variant completely. It gives us more room for optimism.”

For now, the variants don’t appear to be increasing the rate of infection in vaccinated people, but that could change as more data are collected. Read more about breakthrough cases here.

Are there other variants we should be worried about?

The C.D.C. is tracking more than a dozen variants, but only a few qualify as “variants of concern,” which is a public health designation to identify variants that could be more transmissible or have other qualities that make them more of a risk. The main additional variants everyone is talking about right now are the B.1.351, which was first detected in South Africa, and the P.1, which was first identified in Brazil. While there are other variants (including two “California” variants, B.1.427 and B.1.429, and a New York variant, B.1.526) for now, it seems, that the South Africa and Brazil variants (which each account for about 0.5 percent of cases in the United States) are causing the most concern. While a new variant can emerge at any time, existing variants also compete with each other for dominance. One interesting new development: In countries like the United States where B.1.1.7 is dominant, some of the other variants seem to be getting crowded out, making them less of a worry.

Is it true that the variants first identified in South Africa and Brazil can “evade” the vaccines?

There is a concern that the B.1.351 and the P.1 are better at dodging vaccine antibodies than other variants. But that doesn’t mean the vaccines don’t work at all. It just means the level of protection you get from the vaccines against these variants could be lower than when the shots were studied against early forms of the virus. Among the variants, the B.1.351 may pose the biggest challenge so far. It has a key mutation — called E484K, and often shortened to “Eek” — that can help the virus evade antibodies.

Remember that there’s a lot of “cushion” provided by this current crop of vaccines, so even if a vaccine is less effective against a variant, it appears that it’s still going to do a good job of protecting you from serious illness.

How much protection will the vaccines give me against the variant first seen in South Africa?

We don’t yet have precise estimates of vaccine effectiveness against B.1.351, which may be the most challenging variant so far. But studies show that the various vaccines still lower overall risk for infection and help prevent severe disease. A large study of Johnson & Johnson’s one-dose vaccine in South Africa found it was about 85 percent effective at preventing severe disease, and lowered risk for mild to moderate disease by 64 percent. (Distribution of the Johnson & Johnson vaccine has been paused as health officials investigate safety concerns.) There’s less definitive research for the Pfizer and Moderna vaccines against the variant, but it’s believed that these two-dose vaccines still reduce risk of infection against the variant by about 60 percent to 70 percent and still are highly effective at preventing severe disease and hospitalization.

“From everything we know today, there is still protection from the vaccines against the South Africa variant,” said Dr. Stern.

Should I still worry that the vaccines are less effective against some variants?

Part of the problem is that we misinterpret what efficacy really means. When someone hears the term “70 percent efficacy,” for instance, they might wrongly conclude that it means 30 percent of vaccinated people would get sick. That’s not the case. Even if a vaccine loses some ground to a variant, a large portion of people are still protected, and only a fraction of vaccinated people will get infected. Here’s why.

To understand efficacy, consider the data from the Pfizer clinical trials. In the unvaccinated group of 21,728, a total of 162 people got infected. But in the vaccinated group of 21,720, only eight people became infected. That’s what is referred to as 95 percent efficacy. It doesn’t mean that 5 percent of the participants (or 1,086 of them) got sick. It means 95 percent fewer vaccinated people got infected compared to the unvaccinated group.

Now imagine a hypothetical scenario with a vaccine that is 70 percent effective against a more challenging variant. Under the same conditions of the clinical trial, vaccination would still protect 21,672 people in the group, and just 48 vaccinated people — less than one percent — would become infected, compared to 162 in the unvaccinated group. Even though overall efficacy was lower, only a fraction of vaccinated people in this scenario would get infected, most likely with only mild illness.

While far more research is needed to fully understand how variants might dodge some (but not all) vaccine antibodies, public health experts note that an estimate of 50 percent to 70 percent efficacy against a challenging variant would still be considered an adequate level of protection.

“Seventy percent is extremely high,” said Dr. Stern. “Basically what this means is that it’s even more important to get vaccinated. If you have 95 percent efficacy, you can create some form of herd immunity with less people. With 70 percent efficacy, it’s even more important to get vaccinated to protect others.”

Am I going to need a booster shot?

Vaccine makers already are working on developing booster shots that will target the variants, but it’s not clear how soon they might be needed. “In time, you’re going to see a recommendation for a booster,” said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “That booster will elevate everybody’s antibodies and increase durability. The booster will probably be configured to target the South African and Brazil variants.”

Given all these unknowns about the variants, shouldn’t I just stay home even after I’m vaccinated?

Even amid the rise of variants, vaccines will significantly lower your risk for infection and will protect you from serious illness and hospitalization.

People who are vaccinated can socialize, unmasked, with other vaccinated people. While vaccinated people still need to follow local health guidelines about wearing a mask and gathering in groups to protect the unvaccinated, vaccinated people can travel, get their hair and nails done, or go to work without worrying. And vaccinated grandparents can hug their unvaccinated grandchildren. Because there are still some outstanding questions about the risk of vaccinated people carrying the virus, a vaccinated person is still advised to wear a mask in public to protect the unvaccinated — although those guidelines may be updated soon.

“The vaccines protect you, so go get vaccinated — that’s the message,” said Dr. Fauci. “If you’re around other vaccinated people, you shouldn’t worry about it at all. Zero.”

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

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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.”