What people get wrong about herd immunity, explained by epidemiologists

How will the Covid-19 pandemic end? And when?

These have been the biggest questions since the pandemic began earlier this year. The answer likely depends on one routinely misinterpreted concept in public health: herd immunity.

“Herd immunity is the only way we’re going to move to a post-pandemic world,” says Bill Hanage, an epidemiology researcher at Harvard. “The problem is, how do you get to it?”

Typically, the term herd immunity is thought of in the context of vaccination campaigns against contagious viruses like measles. The concept helps public health officials think through the math of how many people in a population need to be vaccinated to prevent outbreaks.

With Covid-19, since we don’t yet have a vaccine, the discussion has centered on herd immunity through natural infection, which comes with a terrible cost. Confusing matters, too, is the persistent and erroneous wishful thinking by some who say herd immunity has already been reached, or will be reached sooner than scientists are saying.

For instance, at a recent Senate hearing, Sen. Rand Paul (R-KY) claimed that New York City has its outbreak under control thanks to herd immunity and the fact that around 22 percent of the city’s residents had been infected.

But Dr. Anthony Fauci of the National Institutes of Health, who was a panelist at the hearing, immediately spoke up to correct the senator: “If you believe 22 percent is herd immunity, I believe you’re alone in that.”

Hypothetically, yes, there are situations under which herd immunity to Covid-19 could be achieved. Manaus, Brazil, an Amazonian city of around 2 million people, experienced one of the most severe Covid-19 outbreaks in the world. At the peak in the spring and early summer, the city’s hospitals were completely full, the New York Times reported.

During this period, there were four times as many deaths as normal for that point in the year. But then, over the summer, the outbreak sharply died down. Researchers now estimate between 44 percent and 66 percent of the city’s population was infected with the virus, which means it’s possible herd immunity has been achieved there. (This research has yet to be peer-reviewed.)

But that’s much higher than 22 percent, and the cost of this herd immunity was immense: Between 1 in 500 and 1 in 800 residents died there, the researchers estimate.

Many more were hospitalized, and still more may suffer long-term consequences of the infection. Similarly, the oft-cited example of Sweden, which has pursued a laxer social distancing strategy than its European peers (partially with the goal of building up herd immunity in younger people, while protecting older residents and trying to keep hospitals from exceeding capacity), has paid a price, too: a much higher death rate than fellow Scandinavian countries.

We’re several months into this pandemic, and herd immunity is still widely misunderstood and being continually misused for partisan goals of discrediting science and scientists. The biggest misconception is that achieving herd immunity through natural infection is a reasonable pandemic response strategy. It’s not. Let’s explain.