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The president was treated with Paxlovid early in his bout with Covid, and while the drug has been credited with great success in suppressing the virus and preventing severe cases and hospitalizations, a number of patients who have taken it have nonetheless tested positive again a few days after the last dose of the five-day regimen.

Initial clinical studies found that only about 1 percent to 2 percent of those treated with Paxlovid, which is made by Pfizer, experienced symptoms again. Subsequent studies of patients have found higher rates, though still in the single digits.

But some doctors and patients have speculated that the rebound rate could be even higher because of anecdotal experiences and because of the characteristics of the highly contagious Omicron subvariants in circulation this summer. Among those who have had a rebound case after taking Paxlovid is Dr. Anthony S. Fauci, the president’s chief medical adviser and a leading figure in the response to the pandemic.

Mr. Biden has been described by aides as eager to get out of isolation and back on the road as the midterm congressional campaign begins to heat up. As a result of his infection, he has had to cancel a number of planned trips and has limited his contact with aides, advisers and others at a time when he has scored some important victories that he would like to promote.