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In the COVID-19 debates, one topic manages simultaneously to be the most and least controversial: “herd immunity.” The concept of herd immunity is uncontroversial, and simply states that when enough members of a population develop immunity to a virus, the bug will fizzle out for lack of people to infect. The controversy, though, echoes every child’s favorite road-trip question: Are we almost there yet?
Two developments have raised the question with new vigor. One is in Washington, where Dr. Scott Atlas has become the newest White House adviser on the coronavirus task force. Atlas advocated for herd immunity in his May Senate testimony, saying, “If infection is still prevalent, socializing among these low-risk groups [younger, healthy adults] presents the opportunity for developing widespread immunity and eradicating the threat.” (His recommendations assume viral spread among low-risk groups will remain largely confined to those groups and won’t have serious long-term health consequences.)
The second development was summarized by a recent Washington Post article: Researchers are now questioning where the threshold for coronavirus herd immunity actually lies. So at what point can we argue that we’ve reached herd immunity?
Viruses spread at different rates in different populations. In a March letter to the editor in the Journal of Infection, a Hong Kong–based team attempted to calculate the coronavirus herd immunity threshold for various countries. The authors started with the basic idea that the more easily the virus spreads within a given population, the further that population must be from the immunity threshold. They based this on estimates of Rt, a figure measuring the “effective reproductive number.” (Think of weeds in gardens: They spread freely in general, but they may spread even better—or worse—depending on their circumstances. An Rt value below 1.0 would mean weeds are decreasing.)
The Hong Kong team estimated the U.S. herd immunity threshold was 69.6 percent, assuming no masks or social distancing—meaning about 7 out of 10 Americans would need to get the coronavirus (and survive) in order to stop its spread.
However, a Brazilian team’s new study (not yet peer reviewed) argues that individual variation in susceptibility or exposure could lower the threshold for herd immunity. That’s an interesting hypothesis, and the logic behind it makes sense up to a point. Clearly people who spend much of their time in crowded indoor places, such as the unfortunate folks who “get every cold that goes around,” are at higher risk than others. But—there’s always a “but,” isn’t there?—the Rt already takes that into account: It’s a population-level figure, and it considers those susceptible people along with the happy souls whose immune systems (or introverted lifestyles) mean they seemingly haven’t caught a virus in years.
The Brazilian team argues that under optimal circumstances, herd immunity could require as little as 20 percent of the population to be immune. Wonderful news if true—but is it? And how close can we get to those optimal circumstances? The fact is, we can do things to change effective reproductive number: America’s calculated Rt value, for example, went from an extreme high of 3.8 in some states down to slightly above 1.0 with social distancing, masking, contact tracing, and full-then-partial shutdowns.
We thus end up in another good news, bad news situation. The good news is that being able to change Rt means we can also temporarily, by all working together, get the benefits of herd immunity—reduced case numbers, reduced deaths, sustained reopening—even without having enough people contract SARS-CoV-2 to obtain true herd immunity. This is how distancing, hand-washing, mask-wearing, contact tracing, and staying home have held Taiwan’s population of 23 million people to a total of seven COVID-19 deaths. The Taiwanese are not unusually immune to the coronavirus. They simply worked together to pull Rt down to a level where the virus couldn’t spread.
The bad news comes if we mistake the good news for a get-out-of-jail-free card. The virus hasn’t changed, and the outbreaks at college campuses show just how easily it still spreads. A vaccine should be mass-produced soon—God willing, within the next six months. Observing that Rt is currently low, or that the infection and death figures are more reassuring now, should encourage us to stay the course with measures like masking and social distancing, not abandon them prematurely.