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Taking the drug, Dr. Auwaerter said, could be like “moving the goal posts” in the course of an infection, suppressing the virus but not clearing it completely. Still, he said, high-risk people should “absolutely” still take the medication.

Dr. John P. Moore, a virologist at Weill Cornell Medicine, said researchers were still lacking correlations between age, risk factors or vaccination status. “I haven’t heard anyone come up with a definitive cause,” he said. “He’s just the unlucky guy in the one out of 20. It’s just a numbers game.”

Dr. Moore said that if data could support such a move, federal regulators might want to consider allowing a longer course of the drug, to definitively rid the body of the virus. “The simplest thing would be to go back on the drug for longer,” he said.

Mr. Biden’s rebound case will complicate his effort to turn his illness into a positive story. As the oldest president in the nation’s history, Mr. Biden, 79, has been eager to show that he remains fit, especially as he forecasts plans to run for a second term in 2024. He continued to work from the White House residence during his first isolation, appearing by video before several groups, and then made a triumphal return to work in person on Wednesday.

Instead of the narrative of beating the virus, however, the president’s rebound case reinforces the unpleasant reality that the pandemic refuses to go away. Although the death toll has fallen dramatically, Covid-19 remains a fact of life for Americans, some of whom have been infected multiple times.

Mr. Biden’s new positive test may also raise questions about his fidelity to precautions against infecting others after returning to the office. Aides said he would wear a mask while with others, but in every public appearance he made since Wednesday, his face remained uncovered.