(NEXSTAR) — Dr. Anthony Fauci, the U.S. government’s top infectious disease expert, said Tuesday that he experienced a recurrence of symptoms after taking the Pfizer’s oral antiviral medication Paxlovid as a treatment for COVID-19.

Fauci, also the current director of the National Institute of Allergy and Infectious Diseases, had initially experienced only mild symptoms upon becoming infected with COVID-19 in mid-June, he said Tuesday at the Foreign Policy’s Global Health Forum.

“When [the symptoms] increased, given my age, I went on Paxlovid for five days and I felt really quite well, really just a bit of rhinorrhea and fatigue,” said Fauci, 81.

After finishing his course of Paxlovid, Fauci claimed he tested negative for COVID-19 on antigen tests for three days in a row. But on the fourth day, he again tested positive. He also began to experience worsening symptoms.

“It was sort of what people are referring to as a ‘Paxlovid rebound,’” Fauci said.

Fauci is currently on another course of Paxlovid, he confirmed.

The phenomenon that Fauci mentioned — also known as COVID-19 rebound — has been acknowledged by the Centers for Disease Control and Prevention. In May, the CDC issued an advisory on the potential recurrence of symptoms and possibly a “new positive viral test after having tested negative” within two to eight days of finishing a five-day course of Paxlovid.

The agency noted, however, that the resurgence of symptoms “may be part of the natural history of SARS-CoV-2 … infection in some persons, independent of treatment with Paxlovid and regardless of vaccination status.”

The CDC added that Paxlovid is still believed to be an effective treatment in the early stages of COVID-19 infection among those at higher risk for severe illness. Still, the agency has not found evidence that a second round of Paxlovid — like the one Fauci is taking — is necessary among those suffering from “COVID-19 rebound.”

Fauci, meanwhile, said Tuesday that he’s feeling “reasonably good,” but “not completely without symptoms.”

He had also said suggested the phenomenon is becoming “more and more typical, the more clinical experience we get.”

The National Institutes of Health first announced Fauci’s COVID-19 diagnosis on June 15, confirming that he would immediately begin isolation and continue to work from home.

“Dr. Fauci will follow the COVID-19 guidelines of the Centers for Disease Control and Prevention and medical advice from his physician and return to the NIH when he tests negative,” the NIH said.