CDC moves away from pandemic goals of collective immunity

Since the early days of the pandemic, there has been a collective goal to end it: to achieve collective immunity. This is when so many people are immune to a virus that they no longer have potential hosts to infect, causing an epidemic.

Many Americans have embraced the new barnyard phrase, and with it, the projection that once 70-80% or 85% of the population is vaccinated against COVID-19, the virus would be gone and the pandemic would be over.

Now the herd is restless. And experts from the Centers for Disease Control and Prevention have set aside herd immunity as a national goal.

The prospects of achieving a clear goal of herd immunity are “very complicated,” said Dr. Jefferson Jones, a physician on the CDC’s COVID-19 Epidemiology Task Force.

“Thinking that we will be able to reach some kind of threshold where there will be no more transmission of infections may not be possible,” Jones admitted last week to members of a panel advising the CDC on vaccines.

The vaccines have been effective enough in preventing cases of COVID-19 that lead to serious illness and death, but none have been proven to be reliable in blocking transmission of the virus, Jones noted. Recent evidence has also clearly shown that the immunity provided by vaccines can weaken within a few months.

The result is that even if the vaccination were universal, the coronavirus would likely continue to spread.

“We would be discouraged” from thinking in terms of “a strict goal,” he said.

For Dr Oliver Brooks, a member of the CDC’s Advisory Committee on Immunization Practices, it was a sobering new message, with potentially worrying effects.

With only 58.5% of all Americans fully vaccinated, “we need to increase” the consumption of COVID-19 vaccines, 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 was concerned that as the CDC rolls back a specific target for herd immunity, it will remove the air from efforts to increase immunization levels.

And if public health officials stop talking about the “herd,” people risk losing sight of the fact that vaccination is not just an act of personal protection but a means of protecting the community.

A public hijacking of the collective immunity promise may also further undermine the credibility of the CDC when it comes to fighting the coronavirus.

On issues ranging from the use of masks to the spread of the virus, the agency has made dramatic about-faces during the pandemic. These reversals were sparked by new scientific findings about the behavior of the new virus, but they also provided enough fuel for COVID-19 skeptics, especially those in the conservative media.

“It’s a science communication problem,” said Dr. John Brooks, CDC’s chief medical officer for the COVID-19 response.

“We said, based on our experience with other diseases, that when you get up to 70 to 80 percent, you often get herd immunity,” he said.

But the SARS-CoV-2 virus did not get the memo.

“He’s got a lot of tricks up his sleeve and he’s challenged us a few times,” 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. Instead of specifying a vaccination target that promises 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 stay. weak for a prolonged period.

“We want clear and easy answers, and sometimes they do exist,” said John Brooks. “But on this one, we’re still learning.”

Collective immunity has never been easier than many Americans claimed, 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 people in the world. increasingly skeptical – and often conspiratorial. – citizens.

This is an idea that emerged about a century ago in the field of livestock medicine. Epidemiologists now calculate it with a standard equation. But like many tools that model a complex process with mathematics, it makes a few simplifying assumptions.

For example, this assumes unrealistic uniformity in the behavior of individuals and groups, and in the ability of the virus to spread from person to person.

So this does not reflect the diversity in population density, lifestyles, modes of transportation and social interactions that make Los Angeles County, for example, so different from Boise County, Idaho. It also ignores the fact that Boise County, where less than 35 percent of adults are fully vaccinated, has no protection against LA County’s 73 percent vaccination rate among adults.

“Humans are not a herd,” Jamieson said.

Public health officials would have been better served by framing their vaccination campaigns around the need for “community immunity,” she said. It would have made people 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 produced a herd immunity estimate of 70-85% relies heavily on the innate transmissibility of SARS-CoV-2. But with the emergence of new viral strains like the Alpha and Delta variants, the ability of the virus to pass from person to person has increased dramatically over the past year.

Additionally, herd immunity calculations assume that when people gain immunity, they stay that way for a known period of time. But it has become clear that neither vaccination nor natural infection provides lasting protection. Recall fire or a “revolutionary” case could, but for how long is still unknown.

This is how science works, said Raj Bhopal, a retired public health professor at the University of Edinburgh who has written about the maddening complexity of herd immunity.

For any agency engaged in public messaging, “it’s very difficult to convey uncertainty and remain bossy,” Bhopal said. “It is a pity that we cannot take the public with us on this road of uncertainty.”

Melissa Healy, Los Angeles Times

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