On ‘herd immunity,’ vaccines, and pandemic whiplash

In mid-March, as the British government dragged its feet on implementing strict coronavirus lockdown measures that it would soon impose anyway, Patrick Vallance, the country’s chief scientific adviser, gave a series of interviews and discussed a concept with which many people were not then familiar: “herd immunity,” or the threshold at which enough members of a given population are immune to an infectious disease that the disease’s spread is controlled. Vallance—and, later, other officials—seemed to suggest that the government’s goal was to allow the virus to circulate while shielding only the most vulnerable against it. As The Atlantic’s Ed Yong put it at the time, the message appeared to...