The government’s top scientific adviser, Patrick Vallance, called it “our last big wave,” and “exit wave” certainly sounds better than third, fourth or fifth wave. The idea seems to be: It is better to get it over with now, to weather the forecast rise in infections in the summer, instead of in the winter, when seasonal flu returns and there will be more pressure on hospitals. “If not now, then when?” Johnson said. Letting the virus go, and reaching a plateau of herd immunity sooner rather than later, is an approach that Britain considered at the very beginning of the pandemic in March 2020 — before the development of successful vaccines — and then abandoned.