Biden Tests Positive for Covid Again in ‘Rebound’ Case

President Biden tested positive for the coronavirus again on Saturday morning, a rebound attributed to the Paxlovid treatment he was taking, but he has not experienced a recurrence of symptoms, the White House physician said.

Mr. Biden “continues to feel quite well,” the physician, Dr. Kevin C. O’Connor, said in a memo released by the White House. “This being the case, there is no reason to reinitiate treatment at this time, but we will obviously continue close observation,” he added.

The positive test, however, means that Mr. Biden will resume “strict isolation procedures,” as Dr. O’Connor put it, in keeping with medical advice. The White House said he would no longer make a planned trip to his home in Wilmington, Del., on Sunday or a work trip to Michigan on Tuesday.

Mr. Biden first tested positive for the virus on July 21. After five days of isolation, he tested negative on Tuesday evening and returned to the Oval Office on Wednesday, declaring that his relatively mild case demonstrated how much progress had been made in fighting the virus. But doctors were watching for signs of Paxlovid rebound and tested him daily since. He tested negative on Wednesday, Thursday and Friday before Saturday morning’s positive result.

Paxlovid rebound has become a source of debate among the scientific community and Covid patients. Initial clinical studies suggested that only about 1 percent to 2 percent of those treated with Paxlovid experienced symptoms again. A June study that has not been peer-reviewed found that of 13,644 adults, about 5 percent tested positive again within 30 days and 6 percent experienced symptoms again.

But the anecdotal accounts of Paxlovid rebound — including a case involving Dr. Anthony S. Fauci, the president’s chief medical adviser — have echoed widely, causing many to wonder whether the reported data was still accurate.

“I think this was predictable,” Dr. Jonathan Reiner, a prominent cardiologist and professor of medicine and surgery at George Washington University Hospital, wrote on Twitter on Saturday after the president’s positive test was disclosed. “The prior data suggesting ‘rebound’ Paxlovid positivity in the low single digits is outdated and with BA.5 is likely 20-40% or even higher.”

In a Centers for Disease Control and Prevention study published last month that examined the drug’s success in protecting people from severe cases of Covid-19, researchers wrote that symptoms from a rebound tended to be milder than those that a patient felt during the primary infection and were unlikely to lead to hospitalization.

The C.D.C. issued an emergency health advisory in May that said people experiencing a rebound “should restart isolation and isolate again” for at least five days, reflecting the agency’s general isolation recommendations for people infected with the virus.

The advisory also said that rebounding did not represent reinfection with the virus or the development of resistance to Paxlovid.

Dr. Ashish K. Jha, the White House’s Covid-19 response coordinator, told reporters when Mr. Biden first tested positive that by looking at Twitter, “it feels like everybody has rebound, but it turns out there’s actually clinical data.”

Large health systems, he said, showed rebounds to be rare, with the percentage of Paxlovid recipients who experienced them “in the single digits.”

“When people have rebound, they don’t end up in the hospital,” Dr. Jha said. “They don’t end up particularly sick.” He added: “Paxlovid is working really well at preventing serious illness, rebound or no rebound, and that’s why he was offered it. And that’s why the president took it.”