Experts: Attaining herd immunity for covid might not be realistic

The light at the end of the covid-19 tunnel might be dimming as the more contagious delta variant surges, vaccination rates wane and experts say the long-sought herd immunity might be beyond reach.

Herd immunity has been a buzzword and, for many, the end goal, since the pandemic began sweeping the globe in late 2019 and early 2020. The idea is that once a majority of a population is protected from infection through exposure or vaccination, the disease will stop spreading because most people will not be susceptible.

Before vaccines became available — when they were a hope rather than a tangible reality — experts put the magical herd immunity number between 60% and 80%. Fears then centered on the idea that many people would need to be exposed to the virus, resulting in unnecessary deaths. Now, experts wonder whether the goal is even attainable.

Herd immunity based on natural immunity, though, is not something the medical world has seen in terms of a global pandemic, said Dr. Graham Snyder, medical director of infection prevention and hospital epidemiology at UPMC.

“The idea that there is one number, a percentage, that we would achieve to have herd immunity is a little bit of a misguided notion,” Snyder said.

He noted that one’s natural immunity to an infection can change over time, and it might not be robust against mutations and variations.

Though imperfect, it was a goal that was, in theory, attainable nonetheless. Experts, including Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases and top medical adviser to the president, believed vaccines would help the country reach that goal.

There is precedent for that type of widespread vaccine-induced immunity, Snyder said, but it takes a strong, targeted vaccination push to “essentially eliminate opportunities for the virus to find somebody who is vulnerable.”

That’s the idea of herd immunity — the majority that can be vaccinated do so to offer protection to the minority that cannot.

It’s become clear, Snyder said, that the chances of that are “very low.”

Fauci himself acknowledged that herd immunity should no longer be the target the country aims for; rather, the focus should be on getting as many people as possible vaccinated and not watching percentages as a benchmark.

“People were getting confused and thinking you’re never going to get the infections down until you reach this mystical level of herd immunity, whatever that number is,” he told The New York Times in May. “That’s why we stopped using herd immunity in the classic sense. I’m saying: Forget that for a second. You vaccinate enough people, the infections are going to go down.”

Dr. Ricardo Franco, an infectious disease professor at the University of Alabama, addressed the idea of chasing herd immunity at a briefing this week by the Infectious Diseases Society of America.

“Delta is here, vaccinations have stalled and breakthrough infections are happening,” Franco said.

Vaccination rates nationwide had begun to stagnate as the summer has worn on, though there has been a slight uptick as more and more people realize the threat posed by the more contagious delta variant.

Polling in July by the Kaiser Family Foundation showed about 3 in 10 American adults remain unvaccinated, including 1 in 10 who say they want to “wait and see” how the vaccine works for others. About 3% say they’ll get the vaccine “only if required.”

Another 14% said they will “definitely not” get a vaccine — a percentage that has remained steadfast since December, when vaccines first became available.

Because roughly 30% of the population remain unvaccinated, that complicates the quest for herd immunity, said Dr. Lee Harrison, the head of the Infectious Diseases Epidemiology Research Unit at the University of Pittsburgh.

“That definitely is complicating the matter because, clearly, to achieve herd immunity, we’re going to have to get a really high proportion of the population immunized,” he said. “Nobody really knows what that magic number is.”

Harrison said the delta variant has raised the bar in terms of what percentage of the population would need to be vaccinated to limit the amount of virus circulating.

“The numbers being thrown out are 80%, 85%, which means we have a long way to go to get there,” he said. “But the basic message hasn’t changed: To get through this pandemic, we’re going to have to get a high proportion of the population immunized.”

It’s not an impossible feat — the world has done it before with viruses such as polio and measles. The World Health Organization puts the immunization rate necessary for herd immunity at 80% for polio and 95% for measles.

“Polio still is out there in several countries, but when it hits a country that has a high rate of immunization, it basically hits a wall and can’t take off,” Harrison said.

Measles is similar, and perhaps more akin to covid-19 because of how very infectious it is, he said.

“With measles, we’re seeing outbreaks in the U.S. only in areas where vaccine coverage falls — often because of misinformation about vaccines,” he said. “So we’ve been able to do it with measles (except for) those exceptions.”

Snyder said the conversations surrounding covid have evolved now to asking whether the virus will go the way of polio and measles or influenza.

“Can we reduce how much virus is around and make it … where it’s generally well-controlled in most places, or is it going to be like influenza, where we basically have no control over it and it has evolved enough that we’re just constantly a step behind and have to catch up with updating our vaccine-induced immunity?” he said.

The outlook, he said, isn’t great.

“We are tilting toward the latter model, where we can expect this virus to be with us for quite some time because we’re always going to be one step behind,” Snyder said. “Having 30% of our population totally unwilling to get vaccinated portends that future.”