So far, the data suggest that Britain’s approach is working — because even a single shot provides strong protection against the virus.

As Dr. Robert Wachter of the University of California, San Francisco, has written, “According to most vaccine experts, delaying shot #2 by a few months is unlikely to materially diminish the ultimate effectiveness of two shots.”

In Britain, the daily number of new Covid cases has fallen by more than 90 percent since peaking in early January. The decline is larger than in virtually any other country. (In the U.S., new cases have fallen 79 percent since January.) Given that the contagious B.1.1.7 variant was first discovered in Britain and is now the country’s dominant virus form, “Britain’s free-fall in cases is all the more impressive,” Wachter told me. “Clearly their vaccination strategy has been highly effective.”