The question of herd immunity is one among the many issues running throughout the debate over the response to COVID-19 for months now.
The data in London, Stockholm, and New York City all have something profound in common: Prevalence of cases reaching or exceeding 20 percent seems to lead to significant burn-out in rapid order, in very similar patterns and timelines (within these three major world cities). A higher infection rate in New York versus other U.S. states remains the most plausible explanation I can find for why New York has seen no case growth increase after their widespread protests and gatherings of about five weeks ago compared to other states with, up to then, a much lower incidence of the disease.
I am more and more convinced (and possibly the markets are, too) that the threshold for herd immunity in a practical sense is much lower than we previously believed. In line with the preceding paragraph, it’s worth reading this article, authored by Paul W. Franks (Professor of Genetic Epidemiology at Lund University) and Joacim Rocklöv (Professor of Epidemiology at Umeå University), which notes the possibility that this lower (20 percent) threshold may exist, although it comes with an important caveat: “it applies to only some communities, depending on interactions between many genetic, immunological, behavioural and environmental factors, as well as the prevalence of pre-existing diseases”.