Herd immunity is not a number (part 2)
By Scott Sumner
COVID-19 researchers and modelers have assumed that at least 60 percent of a population, perhaps 70 percent, would need to be infected with the new coronavirus or vaccinated against it before reaching herd immunity, the point at which the virus can no longer spread widely among a community. Some infectious disease experts are now examining the “hopeful possibility” that far fewer people have to get infected or immunized to achieve herd immunity, The New York Times reports, citing interviews with more than a dozen scientists.
If their new, complicated statistical models are correct, and communities can reach herd immunity with 50 percent or less of people gaining immunity to COVID-19, “it may be possible to turn back the coronavirus more quickly than once thought,” the Times reports. A clear minority of researchers predict as few as 10 or 20 percent of a population developing antibodies to the disease would be sufficient for herd immunity; Stockholm University mathematician Tom Britton calculated the threshold at 43 percent.
I continue to see these extremely misleading articles, which suggest that herd immunity is some sort of stable parameter than can be estimated by scientists. Actually, herd immunity is a function of behavior.
When Covid-19 first appeared it was highly infectious. That led to estimates of 60% or 70% for herd immunity. But unlike with the common cold, societies almost everywhere took precautions to reduce the spread of Covid-19. Even in Sweden there was a great deal of social distancing. Since April, social distancing in New York State has reduced the fatality rate from Covid-19 from almost 1000/day to near zero. And that’s with antibody levels far below 60% (probably closer to 20%)
But if New Yorkers were suddenly to go back to life as usual, Covid-19 cases would likely rise sharply, as New York’s current situation of near herd immunity is predicated on a high level of social distancing and mask wearing.
This does not mean the initial 60% to 70% estimates are correct, even for a population taking no precautions. Scientists now have a better understanding of issues such as natural immunity and heterogeneity in behavior (super-spreaders). So the initial estimates may be incorrect.
Here’s how I look at things. The US has a fatality rate of 533/million, rising fast. Sweden’s has leveled off at 584/million. But New Jersey’s at 1805/million deaths despite lots of social distancing. Indeed I know people in New Jersey who work at home and are extremely unlikely to contract Covid-19 unless they go back to business as usual. Thus even New Jersey is far from herd immunity with no precautions. Therefore it’s unlikely that herd immunity was a sensible solution for the US back in April. The entire US would have likely been hit even harder than New Jersey was hit. (And recall that even official death rates probably undercount the true death toll, at least according to “excess deaths” studies.)
On the other hand, the fatality rates I cite for New Jersey reflect the medical care back in April, when most of the deaths occurred. There is evidence that the death rate for Covid-19 (per actual case) has already fallen sharply in recent months. With improved treatments, it’s very possible that at some point herd immunity will become the optimal strategy. I’m agnostic on that question. But it’s important to think clearly about these issues, and a number of recent news articles have done a disservice to the public by oversimplifying the problem.
PS. People often focus too much on the public policy aspects of this issue. When I said, “Therefore it’s unlikely that herd immunity was a sensible solution for the US back in April” I was not referring to public policy. There’s no way the US government could have adopted herd immunity as a strategy, as the public would have done social distancing on their own, and indeed started doing so even before lockdown were in place.
It’s also worth noting that while Sweden’s economy did better than the Eurozone (so far); its recession was as bad as in Denmark and worse than in Finland. (Norway’s Q2 GDP data is not in yet.) Sweden shows that substantial social distancing was inevitable.