That means if we have a lot of positive antibody tests, we can stop social distancing?
One day, yes, hopefully. But the tests aren’t good enough yet, and we don’t know how many people with antibodies are truly protected from the coronavirus. Antibodies wane over time, and not everyone has the same, lasting response to disease or vaccination—as we’ve seen with diseases like measles and hepatitis B. We don’t know how reliably people who are infected by the coronavirus develop effective antibodies. Figuring that out requires longer-term studies of who gets sick twice, and what sort of antibody response is needed to prevent that.
What percentage of people would need to have antibodies—effective ones, in effective amounts—to completely reopen society?
That comes down to the concept of herd immunity. With a disease like measles, not everyone has complete immunity by way of vaccination (because people’s antibody response to the vaccine waned over time, or because they have refused vaccination in the first place). But except for occasional, local outbreaks, we still collectively have enough antibodies that the virus can’t take hold and cause a pandemic. In the same way, the annual flu season ends as we approach herd immunity to that year’s strain of influenza.
The percentage of people required to reach herd immunity varies based on the virus. For a more contagious virus, we need higher percentages of the population to be immune. Determining that percentage comes down to the “basic reproductive number,” or R0, which is the average number of people who will catch a disease from any given contagious person. There’s still disagreement about what that number is for the new coronavirus, but at the moment many experts estimate that it’s between two and three. In any case, one basic approximation of herd immunity is when you multiply the R0 by the proportion of the population that is not immune and the result is less than one.
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Does all the mask-wearing and social distancing mean we’re going to take longer to get to herd immunity?
Those basic measures can and have helped to lower the R0. If we keep doing them, we could have a relatively low percentage of immune people and still open businesses back up, because the disease would effectively be less contagious. People would get COVID-19, but we wouldn’t be flirting with catastrophic exponential growth.
What’s the best estimate for the percentage of immune people across the United States right now?
Given the caveat I keep repeating—that it’s truly impossible to say—the best guesses are all in the single digits. The rate of positive antibody tests varies widely from place to place. And those tests are limited and we don’t know what they mean. In Chelsea, Massachusetts, a small, early study appeared to show a roughly 30 percent positive rate, while in Santa Clara County, California, the positive rate was about 3 percent. In either case, there’s no evidence that we are near herd immunity. And, again, these studies weren’t meant to measure immunity; they were only meant to measure exposure. Measuring immunity will mean seeing how many of the people with antibodies end up getting sick again.