Everything We Know About Herd Immunity and the Coronavirus

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From the beginning, herd immunity has been dangled as a not-entirely-plausible option for the coronavirus, given how serious (and lethal) the novel coronavirus can be. Now, experts say that members of the population who plan to refuse a vaccine might make the concept even more implausible.

Pre-coronavirus, herd immunity was perhaps best known as an argument made by parents who don’t want to vaccinate their children, and instead rely upon other parents to vaccinate their children, thus reducing the overall risk of infection to their own. Parents in the U.S. have refused to vaccinate their children for measles, believing the nation to have herd immunity, but the current vaccination among children 19–35 months is 90.4 percent, below the 93-95 percent required to prevent measles outbreaks. So measles outbreaks happen.

In the spring, as President Trump hurried the nation to reopen despite growing case numbers, some speculated that he was pivoting to a herd-immunity strategy, whether knowingly or not. But what would that mean for U.S. citizens? Here’s what we know so far.

What does herd immunity mean?

The CDC prefers the term “community immunity,” and defines the concept as a setting in which a high-enough proportion of a population is immune to an infectious disease (either through vaccination or having already had the disease) to make its spread from person to person unlikely. This is an obviously appealing scenario which would allow most people to resume some semblance of “normal” life — if the risk is low enough, businesses and schools can reopen without creating “hot spots” for disease.

When will we achieve herd immunity?

The percentage of people who need to become immune in order to confer herd immunity varies from disease to disease. In some cases, herd immunity begins when just 40 percent of a population can be vaccinated, but other, more contagious diseases require 85 to 90 percent. We don’t yet know exactly how many people need to be vaccinated for the novel coronavirus before we achieve herd immunity, nor do we know what percentage would need to become immune through prior exposure. We don’t even know whether having COVID-19 confers any lasting immunity against reinfection.

We also don’t yet have a vaccine. While various countries and companies around the world are racing to develop one, most estimates say a safe, readily available vaccine before 2021 is extremely unlikely. In April, Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told NBC that it was “in the realm of possibility” that millions of vaccines might be available in January 2021. He noted, however, that accelerated production means heightened risk that the vaccine doesn’t work as it should.

Now, according to the World Health Organization, there are more than 140 vaccines in development by researchers around the world. In an interview with CNN in late June, Fauci said he would “settle” for a vaccine that was 70 percent to 75 percent effective.

What could prevent the U.S. from reaching herd immunity?

Fauci also warned, according to CNN, that herd immunity might not be achieved if too large of a percentage of the population refuses to be vaccinated. When asked about a CNN poll in which one-third of participants said they would not be vaccinated for COVID-19, even if a vaccine was widely available and offered at a low cost, Fauci said that scenario would make it “unlikely” the U.S. would reach herd immunity. “There is a general anti-science, anti-authority, anti-vaccine feeling among some people in this country,” he said, adding that “we have a lot of work to do” in educating people about vaccines.

Is it possible to reach herd immunity before a vaccine?

Gideon Meyerowitz-Katz, an epidemiologist specializing in chronic disease, wrote in The Guardian that a reasonable estimate for COVID-19’s herd immunity threshold is 60-70 percent of the population. (Meyerowitz-Katz also acknowledges that CDC data suggests the number might be more like 85 percent.) If we were to simply let people spread the coronavirus unmitigated until 70 percent are infected, that means roughly 230 million Americans would get sick. If we also assume the best-case fatality rate of 0.3-0.6 percent — a figure which presumes a health-care system with ample resources and space — that means about 690,000 to 1.4 million Americans would die in order to achieve herd immunity. Many, many more would get very seriously ill, and overwhelm those resources we need to keep the fatality rate that low.

And again: even this disastrous outcome relies on the assumption that people who get COVID-19 once can’t ever get it again, and we just do not know that to be true.

Everything We Know About Herd Immunity and the Coronavirus