As yet unpublished work by researchers Sterling Haringa, Sean Trende and Christina Ramirez bolsters this case, with regression analysis suggesting that two factors account for much of the variation in cases between U.S. states: population density and temperature. But with an important caveat: When data was analyzed in March, places with lower temperatures generally had worse covid-19 outbreaks; in June, it was the places with higher temperatures that had the bigger problems, exactly as we’d expect if “indoor gatherings” are the main driver.

Trende told me that when Ohio reopened, he and his wife went to dinner for their anniversary — right about the time when he conducted the analysis for June. “I haven’t been back out since,” he added ruefully.

In other words, felicitous weather may have been impersonating herd immunity. As the weather changes, groups that thought they had herd immunity will discover they don’t, and policies that had looked highly successful will start to fail. It’s not that those policies are useless: Testing, tracing, quarantine, mask-wearing, hand-washing, ventilation and social distancing are all important. But the ever-changing weather can be a powerful tailwind behind those policies — or a headwind slowing them down.