Give Neil Ferguson a break. Nearly two weeks ago Mr. Ferguson, an epidemiologist with Imperial College London, issued a report on Covid-19. Much of the public attention focused on his worst-case projection that there might as many as 2.2 million American and 510,000 British deaths. Fewer paid attention to the caveat that this was “unlikely,” and based on the assumption that nothing was done to control it.

The report was one reason that led Prime Minister Boris Johnson to change policy and lock Britain down. Under the Imperial...