Dr. Anthony Fauci, President Joe Biden’s top medical adviser for the coronavirus pandemic, sought on Wednesday to discourage doubts about the antiviral drug Paxlovid after disclosing that he had suffered what appeared to be a “rebound” of COVID-19 after taking a five-day course of the pills.

“Paxlovid did what it was supposed to do,” Fauci, 81, said, saying that he believed that the treatment, made by Pfizer, kept him out of the hospital when he first tested positive for the virus June 15. He added that he thought the drug also reduced the severity of his initial symptoms.

One thing Paxlovid could not do was keep Fauci from missing his daughter’s wedding. It went on without him in New Orleans two Saturdays ago, when he was sidelined with his initial infection. He participated remotely.

Fauci has been vaccinated against COVID and has received two booster doses. His experience with Paxlovid adds to a growing body of anecdotal evidence about patients whose COVID symptoms improved after they took Paxlovid, and who even tested negative, only to have symptoms occur again a few days later. That is exactly what happened to him, Fauci said; he recently tested positive again after three days of negative tests.

In late May, the Centers for Disease Control and Prevention addressed what it called “the potential for recurrence of COVID-19 or ‘COVID-19 rebound.’ ” It issued an emergency health advisory that described the phenomenon as “a recurrence of symptoms or a new positive viral test after having tested negative” when the initial diagnosis was in the past two weeks.

The agency advised that people experiencing the rebound “should restart isolation and isolate again for at least 5 days” in line with the agency’s isolation recommendations for infected patients, regardless of whether they might have gotten antiviral treatment or isolated after the initial infection.

The advisory noted that “a brief return of symptoms may be part of the natural history of” coronavirus infection in some people, independent of Paxlovid treatment and regardless of vaccination status. It also said that based on some case reports, the rebound did not represent reinfection with the coronavirus, or the development of resistance to Paxlovid.

The CDC said that Paxlovid “continues to be recommended for early-stage treatment of mild to moderate COVID-19 among persons at high risk for progression to severe disease.” The Food and Drug Administration authorized Paxlovid for high-risk people ages 12 and older who weigh at least 88 pounds.

For his part, Fauci tested positive for the coronavirus about two weeks ago after experiencing a “scratchy throat.” He described his initial symptoms as mild, adding that he did not “feel ill.” He began a five-day course of Paxlovid. (“I’m 81 years old, which is a considerable risk factor,” he explained.) After that, he tested negative for three days in a row.

But on the fourth day, he said, he was “surprised and disappointed” to see that he had tested positive again, and he suffered a recurrence of symptoms, he said, that were worse than before, including a low fever, achiness, a runny nose and a “mild cough.”

He called his doctor (you may be wondering who Fauci’s doctor is, but he would not say) and got a prescription for another five-day course of Paxlovid. In its advisory in late May, the CDC noted that there was no evidence that additional treatment was needed with Paxlovid or other COVID therapies when rebound was suspected. But Fauci said taking two courses is relatively common among those who suffer rebounds.

In Pfizer’s application for emergency-use authorization of Paxlovid, the company did suggest that “several subjects appeared to have a rebound in SARS-CoV-2 RNA levels around Day 10 or Day 14.” But the rebound occurred in both those who received Paxlovid and placebo treatment, the company said.

The clinical trial that supported the FDA’s authorization of Paxlovid was conducted in people who were unvaccinated, which has led some experts to say that more data on vaccinated people is needed. Among them is Dr. Carlos del Rio, an infectious disease expert at Emory University, who said that “we urgently need data to know, in people who are vaccinated and over the age of 65 like Tony, who needs it and for how long.”

Fauci said he would like to see a study comparing a five-day course of Paxlovid to a 10-day course, “to see if you can prevent the rebound by giving it for five extra days.”

Fauci finished his second course Wednesday, and said his symptoms were “essentially gone, except for a little bit of a stuffy nose.” He had tested positive Tuesday, he said, but had not yet tested himself again at the time of the interview Wednesday.

“I think there is understandable confusion when people hear about people rebounding,” he said. “Don’t confuse that with the original purpose of what Paxlovid is meant for. It’s not meant to prevent you from rebounding. It’s meant to prevent you from being hospitalized. I’m 81 years old, I was at risk for hospitalization and I didn’t even come close to being sick enough to be hospitalized.”

This article originally appeared in The New York Times.