But much of the ebb and flow of a pandemic cannot be explained by changes in human behavior. That was true with influenza a century ago, and it is true with Covid now. An outbreak often fizzles mysteriously, like a forest fire that fails to jump from one patch of trees to another.

The experience with Alpha in the Midwest this spring is telling…

Even Osterholm said that he had assumed the spring surge would spread from Michigan and his home state of Minnesota to the entire U.S. It did not. It barely spread to nearby Iowa and Ohio. Whatever the reasons, the pattern shows that the mental model many of us have — in which only human intervention can have a major effect on caseloads — is wrong.

Britain has become another example. The Delta variant is even more contagious than Alpha, and it seemed as though it might infect every unvaccinated British resident after it began spreading in May. Some experts predicted that the number of daily cases would hit 200,000, more than three times the country’s previous peak. Instead, cases peaked — for now — around 47,000, before falling below 30,000 this week.

“The current Delta wave in the U.K. is turning out to be much, much milder than we anticipated,” wrote David Mackie, J.P. Morgan’s chief European economist.