How many must be exposed for herd immunity?
Fauci has publicly increased estimate
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 percent 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 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, Fauci tended to cite the same 60 percent to 70 percent estimate that most experts did. About a month ago, he began saying “70 [percent], 75 percent” in television interviews. And last week, in an interview with CNBC News, he said “75 [percent], 80 [percent], 85 percent” and “75 [percent] to 80-plus percent.”
In a telephone interview the next day, 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 Fauci’s conclusions, prominent epidemiologists said that he might be proven right. The early range of 60 percent 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 [percent] to 75 percent,” 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 [percent], 85 [percent].”
“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 [percent] 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 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, 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 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 Morens, 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 percent to 70 percent immunity to stop the disease was based on early data from China and Italy, health experts noted.