The effects of the reopenings will not be immediately apparent, and in the absence of widespread testing, it will be hard to know where the country stands in the fight against the virus.

It takes two or three weeks for the newly infected who become severely ill to need hospitalization. An initial calm may encourage more Americans to drop their guard or more governors to ease restrictions.

“I do worry that people will stay home enough in the states that open earliest so that we don’t immediately see the second wave, and then other states will draw the wrong lessons,” said Dr. Leana Wen, a former health commissioner of Baltimore.

Social distancing has proved effective at interrupting viral transmission in places where it was embraced. But now even formerly terrified New Yorkers, living at the center of the nation’s outbreak, are clearly wearying of it.