Since the start of the pandemic, there has been one common goal to end it: achieving herd immunity. That’s when so many people are immune to a virus that it runs out of potential hosts to infect, causing an outbreak to sputter.
Many Americans embraced the new farming phrase, and with it the prediction that once 70% to 80% or 85% of the population was vaccinated against COVID-19, the virus would disappear and the pandemic would be over.
Now the herd is restless. And experts at the Centers for Disease Control and Prevention have set aside herd immunity as a national goal.
The prospects for meeting a clear target for herd immunity are “very complicated,” said Dr. Jefferson Jones, a medical officer on the CDC’s COVID-19 Epidemiology Task Force.
“Thinking that we can reach some sort of threshold where there will be no more transmission of infections may not be possible,” Jones admitted last week to members of a panel that advises the CDC on vaccines.
Vaccines have been quite effective in preventing cases of COVID-19 leading to serious illness and death, but none have been shown to be reliable in blocking the transmission of the virus, Jones noted. Recent evidence has also made it clear that the immunity afforded by vaccines can wane within months.
As a result, even if vaccination were universal, the coronavirus would likely continue to spread.
“We would discourage” thinking in terms of “a strict goal,” he said.
for dr. Oliver Brooks, a member of the CDC’s Advisory Committee on Immunization Practices, said it was a sobering new message, with potentially worrying effects.
With only 58.5% of all Americans fully vaccinated, “we need to increase the uptake of COVID-19 shots,” said Brooks, chief medical officer of Watts Healthcare in Los Angeles. Unfortunately, he said, Jones’ unexpected admission “almost makes you less motivated to get more people vaccinated.”
Brooks said he is concerned that if the CDC pulls out of a specific herd immunity target, it will be worth the effort to ramp up vaccination levels.
And if public health officials stop talking about the “herd,” people may lose sight of the fact that vaccination is not just an act of personal protection, but a way of protecting the community.
A public tack on the promise of herd immunity could also further undermine the CDC’s credibility when it comes to fighting the coronavirus.
On issues ranging from the use of masks to how the virus spreads, the agency has made some dramatic changes over the course of the pandemic. Those reversals were prompted by new scientific discoveries about how the new virus behaves, but they have also provided ample fuel for COVID-19 skeptics, especially those in conservative media.
“It’s a science communication problem,” says Dr. John Brooks, Chief Medical Officer for the CDC’s COVID-19 response.
“We said, based on our experience with other diseases, when you get to 70% to 80%, you often get herd immunity,” he said.
But the SARS-CoV-2 virus didn’t get the memo.
“It has a lot of tricks up its sleeve and it has repeatedly challenged us,” he said. “It’s impossible to predict what herd immunity will be in a new pathogen until you achieve herd immunity.”
The CDC’s new approach will reflect this uncertainty. Rather than specifying a vaccination target promising an end to the pandemic, public health officials hope to redefine success in terms of new infections and deaths — and they will assume herd immunity has been achieved when both remain low for an extended period of time.
“We want clear, easy answers, and sometimes they exist,” said John Brooks. “But we’re still learning in this area.”
Herd immunity has never been as simple as many Americans have made it out to be, said Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania and an expert on the challenges of communicating science to increasingly skeptical — and often conspiracy. – citizens.
It’s an idea that emerged from veterinary medicine about a century ago. Epidemiologists now calculate it with a standard equation. But like many tools that model a complex process with math, it makes some simplifying assumptions.
It assumes, for example, an unrealistic uniformity in the behavior of individuals and groups, and in the ability of the virus to spread from person to person.
So it doesn’t reflect the diversity of population density, housing situation, transportation patterns, and social interactions that make Los Angeles County, for example, so different from Boise County, Idaho. Nor does it explain the fact that Boise County, where less than 35% of adults are fully vaccinated, is not protected against the 73% vaccination rate among adults in LA County.
“People are not herd,” Jamieson said.
Public health leaders would have done better to base their vaccination campaigns on the need for “community immunity,” she said. That would have led people to think in more local terms — the ones that really matter when it comes to a person’s risk of infection, she added.
Changes in the coronavirus itself have also made herd immunity a moving target.
The calculation that yielded an estimate of herd immunity of 70% to 85% relies heavily on the innate transmissibility of SARS-CoV-2. But with the emergence of new viral strains such as the Alpha and Delta variants, the virus’ ability to jump from person to person has escalated dramatically in the past year.
In addition, herd immunity calculations assume that when people gain immunity, they remain immune for a known period of time. But it has become clear that neither vaccination nor natural infection offers lasting protection. Boostershots or a “breakthrough” case perhaps, but for how long is unknown.
That’s exactly how science works, said Raj Bhopal, a retired public health professor at the University of Edinburgh who has written about the maddening complexities of herd immunity.
For any agency dealing with public posting, “it’s very difficult to convey uncertainty and remain authoritative,” Bhopal said. “It’s a shame we can’t take the public down that road of uncertainty.”