Tagged World Health Organization

Covid-19: How Much Herd Immunity is Enough?

Scientists initially estimated that 60 to 70 percent of the population needed to acquire resistance to the coronavirus to banish it. Now Dr. Anthony Fauci and others are quietly shifting that number upward.

How Much Herd Immunity Is Enough?

How Much Herd Immunity Is Enough?

Scientists initially estimated that 60 to 70 percent of the population needed to acquire resistance to the coronavirus to banish it. Now Dr. Anthony Fauci and others are quietly shifting that number upward.

Dr. Anthony S. Fauci in March. “We really don’t know what the real number is,” he said recently.
Dr. Anthony S. Fauci in March. “We really don’t know what the real number is,” he said recently.Credit…Doug Mills/The New York Times
Donald G. McNeil Jr.

  • Dec. 24, 2020, 5:00 a.m. ET

At what point does a country achieve herd immunity? What portion of the population must acquire resistance to the coronavirus, either through infection or vaccination, in order for the disease to fade away and life to return to normal?

Since the start of the pandemic, the figure that many epidemiologists have offered has been 60 to 70 percent. That range is still cited by the World Health Organization and is often repeated during discussions of the future course of the disease.

Although it is impossible to know with certainty what the limit will be until we reach it and transmission stops, having a good estimate is important: It gives Americans a sense of when we can hope to breathe freely again.

Recently, a figure to whom millions of Americans look for guidance — Dr. Anthony S. Fauci, an adviser to both the Trump administration and the incoming Biden administration — has begun incrementally raising his herd-immunity estimate.

In the pandemic’s early days, Dr. Fauci tended to cite the same 60 to 70 percent estimate that most experts did. About a month ago, he began saying “70, 75 percent” in television interviews. And last week, in an interview with CNBC News, he said “75, 80, 85 percent” and “75 to 80-plus percent.”

In a telephone interview the next day, Dr. Fauci acknowledged that he had slowly but deliberately been moving the goal posts. He is doing so, he said, partly based on new science, and partly on his gut feeling that the country is finally ready to hear what he really thinks.

Hard as it may be to hear, he said, he believes that it may take close to 90 percent immunity to bring the virus to a halt — almost as much as is needed to stop a measles outbreak.

Asked about Dr. Fauci’s conclusions, prominent epidemiologists said that he might be proven right. The early range of 60 to 70 percent was almost undoubtedly too low, they said, and the virus is becoming more transmissible, so it will take greater herd immunity to stop it.

Dr. Fauci said that weeks ago, he had hesitated to publicly raise his estimate because many Americans seemed hesitant about vaccines, which they would need to accept almost universally in order for the country to achieve herd immunity.

Now that some polls are showing that many more Americans are ready, even eager, for vaccines, he said he felt he could deliver the tough message that the return to normal might take longer than anticipated.

“When polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent,” Dr. Fauci said. “Then, when newer surveys said 60 percent or more would take it, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85.

“We need to have some humility here,” he added. “We really don’t know what the real number is. I think the real range is somewhere between 70 to 90 percent. But, I’m not going to say 90 percent.”

Doing so might be discouraging to Americans, he said, because he is not sure there will be enough voluntary acceptance of vaccines to reach that goal. Although sentiments about vaccines in polls have bounced up and down this year, several current ones suggest that about 20 percent of Americans say they are unwilling to accept any vaccine.

Also, Dr. Fauci noted, a herd-immunity figure at 90 percent or above is in the range of the infectiousness of measles.

“I’d bet my house that Covid isn’t as contagious as measles,” he said.

Measles is thought to be the world’s most contagious disease; it can linger in the air for hours or drift through vents to infect people in other rooms. In some studies of outbreaks in crowded military barracks and student dormitories, it has kept transmitting until more than 95 percent of all residents are infected.

Interviews with epidemiologists regarding the degree of herd immunity needed to defeat the coronavirus produced a range of estimates, some of which were in line with Dr. Fauci’s. They also came with a warning: All answers are merely “guesstimates.”

“You tell me what numbers to put in my equations, and I’ll give you the answer,” said Marc Lipsitch, an epidemiologist at Harvard’s T.H. Chan School of Public Health. “But you can’t tell me the numbers, because nobody knows them.”

The only truly accurate measures of herd immunity are done in actual herds and come from studying animal viruses like rinderpest and foot-and-mouth disease, said Dr. David M. Morens, Dr. Fauci’s senior adviser on epidemiology at the National Institute of Allergy and Infectious Diseases.

When cattle are penned in corrals, it is easy to measure how fast a disease spreads from one animal to another, he said. Humans move around, so studying disease spread among them is far harder.

The original assumption that it would take 60 to 70 percent immunity to stop the disease was based on early data from China and Italy, health experts noted.

Epidemiologists watching how fast cases doubled in those outbreaks calculated that the virus’s reproduction number, or R0 — how many new victims each carrier infected — was about 3. So two out of three potential victims would have to become immune before each carrier infected fewer than one. When each carrier infects fewer than one new victim, the outbreak slowly dies out.

Two out of three is 66.7 percent, which established the range of 60 to 70 percent for herd immunity.

The French aircraft carrier Charles de Gaulle arriving in the port of Toulon in April, carrying infected sailors.
The French aircraft carrier Charles de Gaulle arriving in the port of Toulon in April, carrying infected sailors.Credit… Marine Nationale, via Agence France-Presse — Getty Images

Reinforcing that notion was a study conducted by the French military on the crew of the aircraft carrier Charles de Gaulle, which had an outbreak in late March, said Dr. Christopher J.L. Murray, director of the University of Washington’s Institute for Health Metrics and Evaluation.

The study found that 1,064 of the 1,568 sailors aboard, or about 68 percent, had tested positive for the virus.

But the carrier returned to port while the outbreak was still in progress, and the crew went into quarantine, so it was unclear whether the virus was finished infecting new sailors even after 68 percent had caught it.

Also, outbreaks aboard ships are poor models for those on land because infections move much faster in the close quarters of a vessel than in a free-roaming civilian population, said Dr. Natalie E. Dean, a biostatistician at the University of Florida.

More important, the early estimates from Wuhan and Italy were later revised upward, Dr. Lipsitch noted, once Chinese scientists realized they had undercounted the number of victims of the first wave. It took about two months to be certain that there were many asymptomatic people who had also spread the virus.

It also became clearer later that “superspreader events,” in which one person infects dozens or even hundreds of others, played a large role in spreading Covid-19. Such events, in “normal” populations — in which no one wears masks and everyone attends events like parties, basketball tournaments or Broadway shows — can push the reproduction number upward to 4, 5 or even 6, experts said. Consequently, those scenarios call for higher herd immunity; for example, at an R0 of 5, more than four out of five people, or 80 percent, must be immune to slow down the virus.

Further complicating matters, there is a growing consensus among scientists that the virus itself is becoming more transmissible. A variant “Italian strain” with the mutation known as D614G has spread much faster than the original Wuhan variant. A newly identified mutation, sometimes called N501Y, that may make the virus even more infectious has recently appeared in Britain, South Africa and elsewhere.

The more transmissible a pathogen, the more people must become immune in order to stop it.

Dr. Morens and Dr. Lipsitch agreed with Dr. Fauci that the level of herd immunity needed to stop Covid-19 could be 85 percent or higher. “But that’s a guesstimate,” Dr. Lipsitch emphasized.

“Tony’s reading the tea leaves,” Dr. Morens said.

The Centers for Disease Control and Prevention offers no herd immunity estimate, saying on its website that “experts do not know.”

Although W.H.O. scientists still sometimes cite the older 60 to 70 percent estimate, Dr. Katherine O’Brien, the agency’s director of immunization, said that she now thought that range was too low. She declined to estimate what the correct higher one might be.

“We’d be leaning against very thin reeds if we tried to say what level of vaccine coverage would be needed to achieve it,” she said. “We should say we just don’t know. And it won’t be a world or even national number. It will depend on what community you live in.”

Dr. Dean noted that to stop transmission in a crowded city like New York, more people would have to achieve immunity than would be necessary in a less crowded place like Montana.

Even if Dr. Fauci is right and it will take 85 or even 90 percent herd immunity to completely stop coronavirus transmission, Dr. Lipsitch said, “we can still defang the virus sooner than that.”

He added: “We don’t have to have zero transmission in order to have a decent society. We have lots of diseases, like flu, transmitting all the time, and we don’t shut down society for that. If we can vaccinate almost all the people who are most at risk of severe outcomes, then this would become a milder disease.”

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Study of Teenagers Asks: Who’s Happier, Boys or Girls?


Teenagers in Hungary. A W.H.O. report on the well-being of European adolescents found that 15-year-old girls were perhaps the worst off of any group surveyed.

Teenagers in Hungary. A W.H.O. report on the well-being of European adolescents found that 15-year-old girls were perhaps the worst off of any group surveyed.Credit Dave Yoder for The New York Times

Belgian and Polish adolescents are among the least happy in Europe, more than half the teenagers in Greenland smoke, and Eastern European boys are far more likely than girls to have had sex, according to a report from the World Health Organization. But over all, the report found, 15-year-old girls were perhaps the worst off of any group surveyed.

The report on adolescent health and happiness was based on surveys of more than 200,000 young people in 42 countries in Europe and North America. Conducted mainly between September 2013 and June 2014, it was released on Tuesday. (American teenagers did not participate.)

Partly titled “Growing Up Unequal,” the report found that 15-year-old Polish, British and French girls were among those expressing the least satisfaction with their lives. They were the most likely to report a decline in their well-being, and on average, one in five reported poor or fair health. They also displayed an increased dissatisfaction with their bodies, “particularly in western and central European countries, despite actual levels of overweight and obesity remaining stable,” the report said.

W.H.O.’s regional director for Europe, Zsuzsanna Jakab, wrote, “Despite the considerable advances made in the W.H.O. European Region over the decades in improving the health and well-being of young people and recent actions to reduce the health inequalities many of them face, some remain disadvantaged from birth by virtue of their gender.”

Boys reported higher life satisfaction over all. But the report highlighted some elevated risk factors for male adolescents. It found that boys were more likely to engage in physical fights and to experience injury. They smoked tobacco and drank alcohol more often, though in some countries, gender differences in those behaviors were narrowing “as girls adopt behaviors typically regarded as masculine.”

Early tobacco use has declined significantly since 2010, the last time the study was conducted. In many countries, fewer than 20 percent of 15-year-olds reported smoking at least once a week. But in Greenland, 53 percent of girls and 51 percent of boys said they smoked at least weekly.

Drinking was heavily skewed by gender in many countries, with 15-year-old boys in Croatia, Malta, Bulgaria and Italy reporting some of the highest incidences of alcohol use. On average, 16 percent of 15-year-old boys drink alcohol at least once a week; the number for girls is 9 percent.

Frequent marijuana use was also divided along gender lines —in some countries, boys used the drug more than girls — though that factor seemed to matter less where it was most popular. French and Canadian 15-year-olds were among the most enamored of marijuana, with boys and girls similarly reporting they had used it in the previous 30 days. Over all, 15 percent of those surveyed had used.

Boys were more likely than girls to have had sexual intercourse in about half the countries and regions surveyed, particularly in Eastern Europe. Forty percent of male 15-year-olds surveyed in Bulgaria said they had had sex, compared with 21 percent of female Bulgarians of the same age. The numbers were even more skewed in Albania, where 39 percent of boys there said that they had had sex, while 2 percent of girls the same age reported such behavior. Over all, 21 percent of the 15-year-olds surveyed reported having sex.

Apart from surveying the habits and health practices of young people, the latest report added new questions to track the health risks posed to the thousands of young migrants uprooted from their homes, and warned that new communications technology could prove a double-edged sword, particularly when it comes to young people bullying their peers over social media.

“Technology is unquestionably a positive presence in all our lives, but we must remain vigilant to the threats it poses to children and young people,” Jo Inchley and Dorothy Currie, two executives who helped coordinate the study, wrote in its preface.

The report concluded that policy makers must strive to recognize girls’ unhappiness and find structural solutions, and called for efforts to address the “clear gender-difference issue.”

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