A couple herd-immunity studies have come out recently that are worth thinking about. The “herd immunity” threshold, of course, is the percentage of the population that needs to be infected before a virus starts naturally petering out.
The idea here is that when a disease infects people in a population, it works as a sort of “targeted vaccine,” hitting the people who are most susceptible to infection and/or have the most social contacts. If these people are the first to become immune, the virus slows down much more quickly than it would if the same number of people were, say, vaccinated at random.
Many epidemiological models don’t consider this dynamic, and when it’s added, the herd-immunity threshold can be far lower than you’d otherwise expect. These studies seem tentative at this point, but they would explain how a place as dense as New York has managed to keep the virus under control for so long.
One study suggests that “the hardest-hit areas, such as NYC, are close to the persistent heterogeneity herd immunity threshold following the first wave of the epidemic, thereby limiting the spread of infection to other regions during a potential second wave of the epidemic.” Another says the threshold could be as low as 10 to 20 percent, though that seems too low.
It’s great news if the worst-case scenario isn’t as bad as we thought, though getting the entire country to herd immunity still doesn’t sound pleasant.