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...