After completing a course of Paxlovid, the antiviral developed by Pfizer for treating Covid-19, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said he's experiencing a "rebound" of symptoms, a phenomenon that sometimes happens in patients who take the drug.

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Fauci says he's experiencing a rebound of symptoms

Fauci, who is 81, vaccinated, and has received two booster doses, said he initially experienced mild symptoms after testing positive for Covid-19 in mid-June. However, "[w]hen [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," he said.

After completing his course of Paxlovid, Fauci said he tested negative for Covid-19 for three days in a row. However, on the fourth day, Fauci tested positive again and said he started feeling "really poorly … much worse than in the first go around."

"It was sort of what people are referring to as a 'Paxlovid rebound,'" Fauci said, adding that he is on another course of Paxlovid and is feeling "reasonably good" but "not completely without symptoms."

According to Fauci, a rebound of Covid-19 symptoms is becoming "more and more typical, the more clinical experience we get."

However, he clarified that the drug is still effective. "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," he added.

Why do patients taking Paxlovid sometimes experience a rebound of symptoms?

In May, CDC issued a warning saying that patients who take Paxlovid could experience a recurrence of symptoms and a "new positive viral test after having tested negative" within two to eight days after a five-day course of the drug. CDC added that patients who complete a course of Paxlovid and experience a return of Covid-19 symptoms should isolate for an additional five days.

CDC noted however that the recurrence of symptoms "may be part of the natural history of [the coronavirus] … infection in some persons, independent of treatment with Paxlovid and regardless of vaccination status," adding the drug "continues to be recommended for early-stage treatment of mild to moderate Covid-19 among persons at high risk for progression to severe disease."

One study, published recently in Clinical Infectious Diseases, in which researchers from the University of California, San Diego School of Medicine isolated the coronavirus from a patient infected with the BA.2 subvariant and tested the virus' resistance to Paxlovid, found the virus still responded to the drug and had no evidence of mutations that would reduce the effectiveness of Paxlovid. The antibodies from the patient were also still effective at blocking the coronavirus from infecting new cells.

The researchers said they believe symptom rebound is due to insufficient exposure of the drug in the body, meaning not enough of Paxlovid was reaching infected cells to stop viral replication entirely.

"Our main concern was that the coronavirus might be developing resistance to Paxlovid, so to find that was not the case was a huge relief," Aaron Carlin, a study author and assistant professor at UC San Diego, said.

While Pfizer CEO Albert Bourla previously said patients experiencing a viral rebound should take more of the treatment, as Fauci did, FDA officials later said there is "no evidence" that a second course of Paxlovid would benefit patients experiencing a relapse in symptoms. (Bartiromo, Nexstar Media Wire News/The Hill, 6/29; Stolberg, New York Times, 6/30; Weixel, The Hill, 6/29; Mitropoulos, ABC News, 6/29; Carbajal, Becker's Hospital Review, 6/22)