With the peak of the pandemic still weeks away, the time hadn’t come yet for stricter measures, Johnson and his advisers said. They worried about “behavioral fatigue”—if restrictions come into force too early, people could become increasingly uncooperative and less vigilant, just as the outbreak swings into high gear. (As of yesterday, the U.K. has identified 1,391 cases, although thousands more are likely undetected.) And while suppressing the virus through draconian measures might be successful for months, when they lift, the virus will return, said Sir Patrick Vallance, the U.K.’s chief scientific adviser.

To avoid a second peak in the winter, Vallance said the U.K. would suppress the virus “but not get rid of it completely,” while focusing on protecting vulnerable groups, such as the elderly. In the meantime, other people would get sick. But since the virus causes milder illness in younger age groups, most would recover and subsequently be immune to the virus. This “herd immunity” would reduce transmission in the event of a winter resurgence. On Sky News, Vallance said that “probably about 60 percent” of people would need to be infected to achieve herd immunity.

Almost immediately, the supposed plan came under heavy criticism, coupled with confusion that public-health and science advisers would recommend this strategy. Herd immunity is typically generated through vaccination, and while it could arise through widespread infection, “you don’t rely on the very deadly infectious agent to create an immune population,” says Akiko Iwasaki, a virologist at the Yale School of Medicine. And that seemed like the goal.