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