The director of the U.S. Centers for Disease Control and Prevention (CDC), Rochelle Walensky, experienced a COVID-19 rebound after completing a course of Pfizer Inc's COVID-19 pill, the agency said on Monday.

Walensky had experienced mild COVID-19 symptoms less than ten days ago. After completing a course of Pfizer's Paxlovid, and a period of isolation, she had tested negative for the virus, but on Sunday, she tested positive again, CDC added.

Walensky is the third government executive, after U.S. President Joe Biden and National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci, to experience a recurrent viral infection following the first course of Paxlovid.

Some people who took the antiviral drug have suffered from a relapse or a rebound that occurred days after the five-day treatment course had ended, studies have shown.

Last month, the U.S. Food and Drug Administration asked Pfizer to test the effects of an additional course of its antiviral Paxlovid among people who had experienced a rebound in COVID-19 after treatment.

Walensky is up to date on vaccines and isolating at home, where she will participate in planned meetings virtually.