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Colorful illustration of people outside in public.
Illustration: Kieran Blakey

The Nuance

Herd Immunity May Play a Bigger Role Than We Thought

Going forward, hard-hit communities could enjoy a measure of immunity-derived protection

During the first months of the pandemic, so-called herd immunity was a hot topic of conversation among public-health officials, politicians, and the media. The term refers to the percentage of people within a population that must have immunity to a virus in order to effectively retard or block its spread.

Early on, most experts estimated that the herd immunity threshold for SARS-CoV-2 was somewhere north of 60%, meaning that 60% or more of the population would need to develop immunity — either via vaccine or infection — in order to snuff out the virus. But those early estimates were based on incomplete data and overly simplistic statistical models — limitations that most infectious disease experts were quick to highlight at the time.

A lot of the data on the novel coronavirus remains incomplete. For example, researchers still don’t know just how infectious SARS-CoV-2 really is or how long immunity lasts following an infection. These two pieces of information are crucial inputs when it comes to accurately modeling herd immunity thresholds. Further complicating matters: The answers to both of these questions may vary depending on a person’s age, health status, and other factors.

But after months of real-world data, some recent research argues that the herd immunity threshold for SARS-CoV-2 may actually be lower, and maybe a lot lower, than initially suspected. Even if those estimates turn out to be overly sanguine, infectious disease experts say that some areas that experienced high rates of infection earlier this year could now enjoy some immunity-derived protection.

The latest herd immunity estimates and what they really mean

Recently, a team of researchers based in Sweden and the U.K. sought to produce a more accurate model of herd immunity. To do that, they considered the ways in which people of different age groups and social proclivities commingle in real-world settings.

The results of their model, which were published August 14 in the journal Science, determined that a community infection rate of around 43% could, theoretically, be sufficient to produce “a level of population immunity at which disease spreading will decline and stop even after all preventive measures have been relaxed.”

Another recent model, albeit an extreme outlier, placed the herd immunity threshold at 20% or even below it. If that were true, some places — including parts of New York City — would already be at or near the herd immunity threshold.

Most experts say that herd immunity models coming in below 40% are highly dubious. “I have a hard time believing that the herd immunity threshold is as low as 20% or 25%,” says Jeffrey Shaman, PhD, an infectious disease expert and professor of environmental health sciences at the Columbia University Mailman School of Public Health. (Other infectious disease specialists reached for comment echoed his skepticism.)

But, more hopefully, Shaman says that it’s reasonable to think that population immunity below 50% could do the trick. “Given that transmission is imperfect in and of itself, it can be shown mathematically that you can’t really sustain an outbreak at some of these lower levels of immunity,” he says. Compared to the early estimates that suggested two-thirds or more of the population would need immunity to block the virus, Shaman says that newer estimates down in the 40% to 50% range are probably more realistic.

He’s not alone. “I think the exact amount of population immunity required to have these [herd immunity] effects is almost certainly less than the 60% you see in most of these early, very simplistic models,” says William Hanage, PhD, an associate professor and faculty member of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health.

If the newer, lower estimates turn out to be correct, that’s good news for several reasons. It would mean that the world — and particularly its hardest hit locales — is a bit closer than once thought to achieving herd immunity. More importantly, it could also mean that fewer people would need to be vaccinated against SARS-CoV-2 in order to knock the virus out. A recent Gallop poll found that one in three Americans would not get a vaccine against the novel coronavirus if one were available. A lower herd immunity threshold could ensure that this minority group doesn’t throw a lifeline to a virus that medical science has the ability to dispatch.

One aspect of herd immunity that most people misunderstand is that, even when it’s achieved, the virus can still go on infecting and sickening a large number of people.

On the other hand, experts warn that these revised herd immunity forecasts shouldn’t lead to complacency. “I’m concerned that people are saying we’re much closer to herd immunity, so we can relax,” Hanage says.

One aspect of herd immunity that most people misunderstand is that, even when it’s achieved, the virus can still go on infecting and sickening a large number of people. “Herd immunity is the point at which each person who is infected, on average, causes fewer than one onward infection,” Hanage explains. Once this happens, it’s a mathematical certainty that the virus will eventually fade away. But that fading takes time. “You still have the downside of the epidemic curve, so there are still a hell of a lot of people that are going to get infected,” he says.

He also points out that most parts of the U.S. are nowhere close to achieving infection rates above 40%. The very few places that may be close, such as pockets of New York City, have suffered mightily to get there; at least 19,000 New York City residents have died due to Covid-19, and untold thousands are likely dealing with ongoing health complications as a result of a SARS-CoV-2 infection.

“If what happened in New York is the cost of herd immunity, I don’t think anyone should be pointing at that as the way forward,” he says.

A silver lining

Virus-eradicating herd immunity is not likely to come to the rescue in the near future. But experts say that in New York City and other communities that have already experienced high rates of infection, it’s possible — and maybe likely — that infection rates will be somewhat depressed going forward.

“The more people who are recovered and not capable of being infected because they have a robust immune memory to [SARS-CoV-2] and can fight it off, the fewer people there are to get infected,” says Columbia University’s Shaman. These previously infected and immune individuals are like dead ends for the coronavirus, and if enough of them are mixed into a population, they’ll impede the virus’s progress, he says.

“If you look at New York City, you can see that the estimates of antibody prevalence by zip code go from 10% in some neighborhoods to over 50% at one in Queens,” he says, referring to tests that are designed to identify previous SARS-CoV-2 carriers. “As we get into winter and people are indoors and schools are open, we will have to see what level the outbreak reaches in these places.”

Assuming that these prior carriers are now immune — at least for some time — they contribute to a variable that virus modelers call the “effective reproduction rate.” This is a measure of the number of people within a community who are likely to become infected if exposed to someone who can transmit the virus. “The effective reproduction rate is changed by how many people get immunity,” says Paul Delamater, PhD, an assistant professor at the University of North Carolina who specializes in mapping population health issues and outcomes. “So if the infection runs through the entire population,” he says, “eventually you could get to an effective rate of zero because there are no more people to pass on the infection to.”

“If what happened in New York is the cost of herd immunity, I don’t think anyone should be pointing at that as the way forward.”

Delamater’s scholarly work has examined the complexity of herd immunity models. For months, he’s been maintaining a nonprofit website that tracks SARS-CoV-2 within his home state of North Carolina. He says that if people who have been infected are immune from the virus — which seems likely in most cases despite some isolated reports of possible reinfections — then this should reduce the effective reproduction rate. But based on his modeling, he says that virus-blocking behavioral changes such as social distancing and donning a mask tend to lower the effective reproduction rate “by many more orders of magnitude.”

Likewise, Hanage says that, for the foreseeable future and across all U.S. communities, it’s essential that people continue to wear masks and follow the safety guidelines that are now proven to slow the spread of SARS-CoV-2.

“People and politicians can cherry-pick models that say what they want to hear, but I think it’s better to consider a range of possibilities,” he says. “I don’t want to go back to where we were in the spring.”

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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