CDC shifts pandemic goals away from reaching herd immunity

Since the beginning of the pandemic there has been a common goal: achieving herd immunity . This is when a virus becomes so resistant that there are no more hosts for it to spread, and an outbreak occurs.

Many Americans embraced the novel farmyard phrase, and with it, the projection that once 70% to 80% or 85% of the population was vaccinated against COVID-19, the virus would go away and the pandemic would be over. Now the herd is restless. Experts at the Centers for Disease Control and Prevention (CDC) have made herd immunity a national goal.

The prospects for meeting a clear herd-immunity target are “very complicated,” said Dr. Jefferson Jones, a medical officer on the CDC’s COVID-19 Epidemiology Task Force.

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“Thinking that we’ll be able to achieve some kind of threshold where there’ll be no more transmission of infections may not be possible,” Jones acknowledged last week to members of a panel that advises the CDC on vaccines.

Vaccines have been quite effective at preventing cases of COVID-19 that lead to severe illness and death, but none has proved reliable at blocking transmission of the virus, Jones noted. Evidence has shown that vaccines’ immunity can diminish in just a few months.

The result is that even though vaccination was universal, the coronavirus will likely continue to spread. He stated that “a strict goal” would discourage him from thinking about it.

To Dr. Oliver Brooks ,, a member the CDC’s Advisory Committee on Immunization Practices, this was a disturbing new message with potentially troubling consequences.

With just 58.5% of all Americans fully vaccinated, “we do 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.”

A person checks a woman's temperature with a forehead reader

Dr. Oliver Brooks, center, looks on as Lucy Arias checks a patient’s temperature at a COVID-19 screening station outside the Watts Health Center in Los Angeles.

(Irfan Khan / Los Angeles Times)

Brooks said he worries that as the CDC backs off a specific target for herd immunity, it will take the air out of efforts to run up vaccination levels.

If public health officials stop talking of the “herd”, people might lose sight of the fact vaccination is more than a personal defense. It’s a way to protect the entire community.

A public tack away from the promise of herd immunity may also further undermine the CDC’s credibility when it comes to fighting the coronavirus. The agency has had to make some major changes in the face of numerous issues, including the spread of the virus and the use of masks. Those reversals were prompted by new scientific discoveries about how the novel virus behaves, but they’ve also provided ample fuel for COVID-19 skeptics, especially those in conservative media.

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

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

A gray cat sits on a man's shoulders

Dr. John Brooks, chief medical officer for the CDC’s COVID-19 response, working from his home office with his cat, Cosmo.

(Centers for Disease Control and Prevention)

But the SARS-CoV-2 virus didn’t get the memo.

” “It has many tricks up its sleeves, and it’s repeatedly challenged me,” he stated. “It’s difficult to predict what herd immunity will look like in a new pathogen before you reach herd immunity.” The CDC’s new approach will reflect that uncertainty. Public health officials are not going to set a vaccine target that will end the pandemic. Instead, they want to define success in terms new infections and deaths. They’ll conclude that herd immunity is achieved when both numbers remain low for a sustained time.

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

Herd immunity was never as simple as many Americans 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-minded — citizens.

It’s an idea that emerged about a century ago from the field of livestock medicine. Epidemiologists now calculate it with a standard equation. It makes some assumptions, just like other tools that model complex processes with math.

It assumes an unrealistic uniformity of behavior and virus spread between people. It doesn’t take into account the differences in population density, living arrangements and transportation patterns, as well as social interactions, that make Los Angeles County so different from Boise County. Nor does it account for the fact that Boise County, where less than 35% of adults are fully vaccinated, gets no protection from L.A. County’s 73% vaccination rate among adults. Jamieson stated that

“Humans do not belong to a group.

Public health officials would have been better served if they framed their vaccination campaigns around “community immunity.” She said that this would have encouraged people to consider local factors, which are the most important when it comes down to an individual’s chance of contracting infection.

Changes to the coronavirus have made herd immunity an evolving target.

The calculation that produced a herd immunity estimate of 70% to 85% rests heavily on the innate transmissibility of SARS-CoV-2. But with the emergence of new viral strains like the Alpha and Delta variants, the virus’ ability to jump from person to person has escalated dramatically in the last year.

Herd immunity calculations assume that people who have acquired immunity will remain immune for a certain period of time. But it’s become clear that neither vaccination nor natural infection confers lasting protection. Although booster shots and “breakthrough” cases might be effective, it is not known how long they will last.

That’s just the way 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.

It is very difficult to communicate uncertainty and still be authoritative for any agency involved in public messaging. Bhopal stated that. “It’s a pity we can’t take the public along with us on that road of uncertainty.”

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