CDC shifts pandemic goals away from reaching herd immunity

From the earliest days of the pandemic, there has been a common goal to end it: achieving herd immunity. Then so many people are immune to a virus that it runs out of potential hosts to infect and an outbreak erupts.

Many Americans welcomed the novel farm phrase and with it the prediction that the virus would go away and the pandemic would be over once 70% to 80% or 85% of the population was vaccinated against COVID-19.

Now the herd is restless. And experts at the Centers for Disease Control and Prevention have made herd immunity a national goal.

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

“To think that we’re going to be able to get to some sort of threshold where there’s no transmission of infection might not be possible,” Jones admitted last week to members of a panel advising the CDC on vaccines.

Vaccines have proven very effective at preventing cases of COVID-19, which lead to serious illness and death, but none have proven reliable at blocking transmission of the virus, Jones noted. Recent evidence has also made it clear that the immunity provided by vaccines can wear off in a matter of months.

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

“We would advise against” 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 sobering new news with potentially worrying implications.

With only 58.5% of Americans fully vaccinated, “we need to increase uptake of COVID-19 vaccines,” said Brooks, chief medical officer of Watts Healthcare in Los Angeles. Unfortunately, he said, Jones’ unexpected admission “makes you almost 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 Watts Health Center in Los Angeles.

(Irfan Khan/Los Angeles Times)

Brooks said he fears that if the CDC withdraws a specific herd immunity target, it will thwart efforts to increase vaccination levels.

And when public health officials stop talking about the “herd,” people could lose sight of the fact that vaccination is not just an act of personal protection, but a way to protect the community.

A public reversal from the promise of herd immunity may also further undermine the CDC’s credibility in fighting the coronavirus.

On issues ranging from mask use to the spread of the virus, the agency has made some dramatic about-faces as the pandemic has unfolded. These reversals were prompted by new scientific discoveries about the behavior of the novel virus, but they’ve also provided plenty of fuel for COVID-19 skeptics, especially those in conservative media.

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

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

A gray cat sits on a man's shoulders

dr John Brooks, the CDC’s chief medical officer for the COVID-19 response, works 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.

“He has a lot of tricks up his sleeve and has always challenged us,” he said. “It’s impossible to predict what herd immunity will be to a new pathogen until you’ve achieved herd immunity.”

The CDC’s new approach will reflect this uncertainty. Rather than set a vaccination target that promises an end to the pandemic, public health officials are hoping to redefine success in terms of new infections and deaths – and they will suspect herd immunity has been achieved if both remain low for an extended period.

“We want clear, simple answers, and sometimes there are,” said John Brooks. “But we’re still learning here.”

Herd immunity has never been as easy 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 science communication to increasingly skeptical — and often conspiracy-oriented — citizens.

It’s an idea that originated in livestock medicine about a century ago. Epidemiologists now calculate it using a standard equation. But like many tools that model a complex process using mathematics, some simplifying assumptions are made.

For example, it assumes 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 in population densities, housing types, transportation patterns, and social interactions that make Los Angeles County so different from Boise County, Idaho, for example. It also doesn’t take into account the fact that Boise County, where fewer than 35% of adults are fully vaccinated, gets no protection from LA County’s 73% adult immunization rate.

“Humans are not a herd,” Jamieson said.

Public health leaders would have been better served by framing their immunization campaigns around the need for “community immunity,” she said. That 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, which resulted in a herd immunity estimate of 70% to 85%, relies heavily on the innate transmissibility of SARS-CoV-2. But with the emergence of new strains of the virus, such as the alpha and delta variants, the virus’ ability to jump from person to person has increased dramatically in the past year.

Additionally, herd immunity calculations assume that people who gain immunity remain immune for a known period of time. But it has been shown that neither vaccination nor natural infection offer permanent protection. Booster shots or a “breakthrough” case may be possible, but for how long is still unknown.

That’s how science works, said Raj Bhopal, a retired public health professor at the University of Edinburgh who has written about the insane complexities of herd immunity.

For any agency that deals with public messaging, “it’s very difficult to convey uncertainty and remain authoritative,” Bhopal said. “It’s a shame that we can’t take the public with us on this path of uncertainty.” CDC shifts pandemic goals away from reaching herd immunity

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