Anthony Fauci, the chief medical adviser to President Joe Biden, suffered a rebound of COVID-19 symptoms after taking Pfizer Inc.’s antiviral Paxlovid.

Despite being fully vaccinated and twice-boosted, Fauci contracted COVID earlier this month and was prescribed Paxlovid due to his age – 81 – which puts him at high risk of developing complications.

“After I finished the five days of Paxlovid, I reverted to negative on an antigen test for three days in a row,” Fauci said Tuesday during an event at Foreign Policy’s Global Health Forum. “And then on the fourth day, just to be absolutely certain, I tested myself again. I reverted back to positive.”

Scores of patients have reported the phenomenon, often called COVID rebound or Paxlovid rebound, of returning symptoms after taking a full course of Pfizer’s drug. While Pfizer Chief Executive Officer Albert Bourla said in May that doctors could prescribe a second course of treatment to such patients, US drug regulators have said there’s no evidence that a repeat will help. However, Fauci said he started taking a second course of Paxlovid after experiencing symptoms “much worse than in the first go around.” Now near completion of the five-day oral treatment, he said he was still enduring symptoms but felt “reasonably good.”

Researchers at Pfizer and elsewhere continue to study the symptom rebound phenomenon.

Pfizer this month stopped enrollment in a study of the drug in patients who aren’t at high risk of severe disease after the pill didn’t alleviate symptoms. The study failed to demonstrate that the drug reduced COVID symptoms among relatively healthy patients and wasn’t able to show a statistically significant reduction in hospitalization and death.