Fauci says he believes Paxlovid kept him out of the hospital, even though he tested positive again.

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 Dr. 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 C.D.C. noted that there was no evidence that additional treatment was needed with Paxlovid or other Covid therapies when rebound was suspected. But Dr. 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 F.D.A.’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 in an interview 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.”

Dr. 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.”

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