Still, only 62 percent of Americans are fully vaccinated, and the nation’s medical infrastructure is dangerously frayed two years into the pandemic as hospitals contend with staff shortages fueled by burnout and early retirements.

Public health experts warned that the most severe disruptions could still be ahead.

Past surges of the coronavirus have been regional, allowing states to reallocate resources like monoclonal antibodies, while this wave threatens to overtake the country at once, said Michael Osterholm, a professor and director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“With this one, all 50 states are in the soup at the same time. It’s like every state is being hit by a viral hurricane,” he said.

Dr. Osterholm predicts that in the next three to five weeks, a substantial share of health care workers will get infected and be unable to work, straining an overburdened system. “We’re already stretched so thin,” he said.

Scientists said those staffing shortages — in hospitals and nursing homes, but also in restaurants, retail stores and airline workforces — had increased the urgency of re-evaluating isolation periods. And at-home rapid tests had given people the ability to get a rough, if imperfect, measure of whether they were contagious.

But the scarcity of those tests over the holiday period made loosening isolation recommendations significantly more fraught, scientists said. Several experts, including Dr. Jha, said that two negative rapid tests on consecutive days would offer more reassurance that someone was not contagious.

A lack of widespread access to tests, some scientists said, had made changing isolation policies more difficult.