Nevertheless, he said, immunity alone probably doesn’t explain the across-the-board collapse in British cases, because the vaccine coverage is so much spottier than that trend would suggest. Instead, you’d likely need widespread behavior change within those groups, particularly among the young males who had really driven the surge. He, too, cited school closures, as well as the end of the European soccer championships, which produced so much congregating among young men. (In fact, everyone I spoke to mentioned the Euros, with one scientist suggesting it elevated the British “R” figure by as much as 0.4 — a remarkable surge, given that the R boost of last year’s Thanksgiving travel surge in the U.S. may have been only 0.09, at least according to Youyang Gu, the 2020’s best pandemic modeler.) Kucharski also credited the success of the country’s contact-tracing app. In the week ending July 21, he said, 700,000 notifications were sent out via app. As a result of additional precaution, he said, “the susceptibility network is much more fragmented,” with the behavioral changes by those groups making a much bigger difference than they would have a year ago — the effect of vaccines effectively amplifying the importance of the behavior of the unvaccinated in determining the shape and rate of spread.
The most encouraging hypothesis I heard was that the country had reached some version of herd immunity, with perhaps 80 percent of the population having attained protection against the disease through exposure or vaccination — a level that, holding factors like social behavior and weather constant, might be sufficient to permanently suppress the disease spread. Other estimates have suggested that as many as 92 percent of the U.K. has antibodies. Unfortunately, American estimates of infections are much more hodgepodge — a sign of how poorly the disease is being tracked by the country’s public-health infrastructure, which, believe it or not, recently stopped counting breakthrough cases, just before the Delta surge — but probably about one-third of the country has already gotten the disease here in the U.S. Just under half of the total population has been fully vaccinated, compared to 57 percent in the U.K., though of course there is significant overlap between the groups.
“Herd immunity” has been described in some strange ways during the pandemic. It is not a light switch that goes on at a certain point, completely stopping the disease, but a growing level of protection in the population at large that, as it accumulates, slows spread. But that is one reason the recent British trajectory is so striking — a rapid ascent is followed, not by a plateau or a gradual downslope, but by a just-as-rapid descent. It is almost like a light switch was flipped, all the more conspicuous for happening just two days after the country’s pandemic restrictions were, amidst much consternation, lifted.
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