Biden Tests Positive for Covid Again in ‘Rebound’ Case

President Biden tested positive for the coronavirus again on Saturday morning, becoming the latest example of a rebound case after taking the Paxlovid treatment that has otherwise been credited with broadly impressive results in fighting the virus and suppressing its worst effects.

“The president has experienced no re-emergence of symptoms, and continues to feel quite well,” Dr. Kevin C. O’Connor, the White House physician, said in a memo released by the press office. “This being the case, there is no reason to reinitiate treatment at this time, but we will obviously continue close observation.”

The “‘rebound’ positivity,” as Dr. O’Connor termed it, meant that Mr. Biden was forced to resume “strict isolation procedures” in keeping with medical advice. The White House announced that the president would no longer travel to his home in Wilmington, Del., on Sunday as planned nor make a scheduled visit to Michigan on Tuesday to promote newly passed legislation supporting the domestic semiconductor industry.

Mr. Biden played down the development. “Folks, today I tested positive for COVID again,” he wrote on Twitter. “This happens with a small minority of folks. I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon.”

The White House later posted a video of the president on the Truman Balcony with his dog Commander and he appeared well. “I’m feeling fine,” he said. “Everything’s good.”

Mr. Biden first tested positive for Covid-19 on July 21 and experienced a sore throat, runny nose, cough, body aches and fatigue. 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 that has killed more than one million Americans.

But doctors were watching for signs of a rebound case and made sure to keep testing him every day. He tested negative on Wednesday, Thursday and Friday before receiving a positive antigen result on Saturday morning.

Paxlovid rebound has become a source of debate within the scientific community and among Covid patients. Initial clinical studies of the drug, which is made by Pfizer, suggested that only about 1 percent to 2 percent of those treated with Paxlovid experienced symptoms again. A study published in June that has not yet been peer-reviewed found that of 13,644 adults, about 5 percent tested positive again within 30 days and 6 percent experienced symptoms again.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 as the new and much more contagious BA.5 subvariant sweeps through communities and reinfects even patients who recently recovered from Covid-19.

“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. He added that “the prior data suggesting ‘rebound’ Paxlovid positivity in the low single digits is outdated” and that the real number was likely significantly higher.

Either way, experts stressed that Paxlovid had been notably successful in preventing more severe Covid-19 illnesses and hospitalizations. And a Centers for Disease Control and Prevention study published in June reported that symptoms from a rebound tended to be milder than during the primary infection and unlikely to lead to hospitalization.

“While we continue to monitor real-world data, we remain very confident in the treatment’s effectiveness at preventing severe outcomes from Covid-19,” Amy Rose, a Pfizer spokeswoman, said in a statement on Saturday.

The C.D.C. issued an emergency health advisory in May that said people experiencing a rebound case “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 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” suggesting otherwise. Moreover, he said, “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.”

Dr. Paul G. Auwaerter, the clinical director in the infectious diseases division at the Johns Hopkins University School of Medicine, said it was unlikely that Mr. Biden, who has been fully vaccinated and boosted twice, would become seriously ill. He added that scientists were working to explain why some people experience a rebound of the virus.

Among his Covid-19 patients experiencing a rebound case, Dr. Auwaerter said, many of them have had the recent Omicron subvariants. None has been hospitalized while rebounding. Those highly infectious and vaccine-evasive forms of the virus, he added, can cause people to test positive for longer.

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.

Aides said that he was socially distant from others and that he was cautious to avoid exposing aides, Secret Service agents and members of the household staff. The White House Medical Unit found that 17 people had been in close contact with Mr. Biden before his initial positive test, but as of Wednesday none had tested positive.

While the president did not wear a mask in the video on Saturday, a photograph released by the White House showed him wearing one as he signed a disaster declaration responding to flooding in Kentucky.

Dr. Auwaerter said Mr. Biden might not have put others at great risk in the last few days even without wearing a mask, since he was being tested for the virus regularly and was testing negative. For those not testing as regularly, he said, it would be prudent to continue wearing a tightfitting and high-quality mask, particularly around high-risk people, because of how infectious Omicron subvariants can be.

But the new positive test will also set back Mr. Biden’s efforts to get back on the road to promote his agenda and campaign for Democrats facing an uphill struggle to keep control of both houses of Congress in this fall’s midterm elections.

The president, whose approval rating stood at only 33 percent in a New York Times/Siena College poll in July, has been described as eager to travel the country after a spate of foreign trips, but the renewed isolation will delay that further.