The U.S. Food and Drug Administration (FDA) has ordered Pfizer Inc to test the effects of an additional course of its antiviral Paxlovid among people who experience a rebound in COVID-19 after treatment, the regulator said on Friday.

The drugmaker must produce initial results of a randomized controlled trial of a second course of the antiviral by Sept. 30 next year, the FDA told Pfizer in a letter dated Aug. 5.

The directive follows reports of recurrent viral infection or symptoms, or both, after the first course, including in President Joe Biden and National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci.

The incidents, which Pfizer says are rare, prompted the FDA to start talks with the company about the trial in May.

The regulator said a protocol for the study is expected to be finalized this month.

Pfizer is "working with the FDA to finalize a protocol to study patients who may be in need of retreatment" and will provide details when available, a company spokesperson said.