Pfizer Told by FDA to Test Second Paxlovid Course for Rebound

Pfizer Inc. was ordered by US regulators to test the effects of an additional course of Paxlovid among people who experience a rebound in Covid-19 after treatment, putting the drug through further study as reports of the mysterious phenomenon continue to mount.

The company must produce initial results of a randomized controlled trial of a second course of the antiviral by Sept. 30, 2023, according to a letter from the regulator to Pfizer dated Aug. 5. The letter, which describes changes in Paxlovid’s emergency authorization, was confirmed by the Food and Drug Administration in an email.

The drugmaker is “working ...