Summer

weather, including ultraviolet light, had some effect on the virus and its spread but said “social interventions have by far the most important effect.”

And a short report from the National Academies of

Sciences, Engineering and Medicine concluded that summer was not likely to slow the virus significantly.

All of the studies acknowledge uncertainty and limitations in their findings. And none diminish the primacy of public health measures. Juni said that given the effectiveness of social restrictions, school-opening strategies should be very carefully planned and tested.

“We can’t have schools closed for more than a year and a half,” he said, but it is not yet known how best to reopen schools and what policies should be followed. Mistakes could mean that openings backfire, with devastating consequence for spread of the disease.

RICH PEDRONCELLI/AP Rafters hit the cool waters of the American River on May 9 in Rancho Cordova, California.