How Much Herd Immunity Is Enough?

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

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. 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». Fauci acknowledged that he had slowly but deliberately been moving the goal posts. 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.

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».

« 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.

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. Fauci’s. «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. Chan School of Public Health.

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. 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. 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. 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. 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. 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. Further complicating matters, there is a growing consensus among scientists that the virus itself is becoming more transmissible.

Lipsitch agreed with Dr. Fauci that the level of herd immunity needed to stop Covid-19 could be 85 percent or higher. Lipsitch emphasized. 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. Lipsitch said, «we can still defang the virus sooner than that».

« If we can vaccinate almost all the people who are most at risk of severe outcomes, then this would become a milder disease».

Read more.

Source: Donald G. McNeil Jr. | The New York Times