Each state, each city has its own crisis driven by its own risk factors: vacation crowds in one, bars reopened too soon in another, a revolt against masks in a third.

“We are in a worse place than we were in March,” when the virus coursed through New York, said Dr. Leana S. Wen, a former Baltimore health commissioner. “Back then we had one epicenter. Now we have lots.”…

While there may be no appetite for a national lockdown, local restrictions must be tightened when required, the researchers said, and governors and mayors must have identical goals. Testing must become more targeted…

By the end of summer, the administration hopes to start using “pooling,” in which tests are combined in batches to speed up the process.

But the method only works in communities with lower infection rates, where large numbers of pooled tests turn up relatively few positive results. It fails where the virus has spread everywhere, because too many batches turn up positive results that require retesting.