It’s easy to see the appeal in the way Sweden has, thus far, responded to the coronavirus pandemic: Most businesses there have stayed open, and most people have been free to go about their lives as usual. They have employed what’s known as a “herd immunity” strategy, allowing enough lower-risk people to get infected that the risk of infection is eventually reduced for everyone. (Though British government officials have since denied it was ever their official strategy, they initially did the same — though they soon reversed course, and Prime Minister Boris Johnson became seriously ill himself.)
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.
From the beginning, herd immunity has been dangled as a not-entirely-plausible option, given how serious (and lethal) the novel coronavirus can be. But now, as President Trump hurries the nation to reopen despite growing case numbers, some have speculated that he’s pivoting to a herd-immunity strategy, whether knowingly or not. But what does 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 85-90 percent. We don’t yet know 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. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, recently told NBC that it was “in the realm of possibility” that millions of vaccines might be available in January 2021. He also noted, however, that accelerated production means heightened risk that the vaccine doesn’t work as it should.
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.