By sifting for seasonal patterns across individual states, Jalal and his collaborators found very robust results. They argue that the calendar of COVID in North America has already taken shape, in the form of three repeating waves like the ones that swept the continent in 2020: one starting in New England and eastern Canada in the spring, the second traveling north from Mexico over the summer, and the third emanating in all directions from the Dakotas during the fall. In keeping with that idea, their paper predicted a summer 2021 wave in the South, and a fall 2021 wave in the north-central states—which is more or less exactly what happened.

This three-peaked seasonality, if it’s real, would seem to make COVID an outlier, at least compared with single-season diseases like the flu. But if COVID really is driven more by seasonal changes than factors such as masking and vaccination rates, no community should expect to see a surge more than once a year. The disease would still behave like the flu on a local level, in the sense that each place would see one peak season every year—even while the country overall had three.

This pattern may sharpen in the next few years. David Fisman, an epidemiologist at the University of Toronto, told me that the patterning of past pandemics has tended to follow a sort of script: chaos, then seasonality, then less-destructive chaos.