Over the last few months, coronavirus contrarians and those hoping for more optimistic news have spent a fair amount of time focusing on two particular areas of apparent weirdness. The first is how the heterogeneity of the disease’s spread affects the threshold of herd immunity — the point at which enough people in a community have been exposed to the disease that it can no longer really spread. If that point arrives sooner than was projected early in the pandemic, when a rough, conventional-wisdom calculation held that herd immunity would require between 60 percent and 80 percent of a community to be exposed, it would mean that the “end” would be coming much sooner, too — and perhaps that some hard-hit places, like New York City, had already reached a point of relative safety.
The second is how the heterogeneity of the immune response affects disease outcomes across a population: who gets mildly sick, who gets very sick, who recovers, and who dies. Some of those looking for good news have been especially focused on the heterogeneity of the T-cell aspects of the immune response, because a handful of studies have found that a quite significant number of people unexposed to the coronavirus nevertheless exhibited what are called “cross-reactive” T-cell immune responses to the disease. In other words, you didn’t necessarily need to catch COVID-19 for your T-cells to know how to fight it, because previous exposure to similar coronaviruses (chiefly the common cold) had already taught your immune systems how to respond to this one.
In both cases, the picture remains somewhat uncertain; all research into COVID-19 and SARS-CoV-2 is very young. But the early returns suggest that while the maximalist interpretation of each hypothesis is not very credible — herd immunity has probably not been reached in many places, and cross-reactive T-cell response almost certainly does not functionally immunize those who have it — more modest interpretations appear quite plausible. It may well be the case that some amount of community protection kicks in below 60 percent exposure, and possibly quite a bit below that threshold, and that those who exhibit a cross-reactive T-cell immune response, while still susceptible to infection, may also have some meaningful amount of protection against severe disease.