U.S. government researchers are planning studies of how often and why coronavirus levels rebound in some Covid patients who have completed a five-day course of treatment with Pfizer Inc.’s Paxlovid.

“It is a priority,” said Clifford Lane, deputy director for clinical research at the National Institute of Allergy and Infectious Diseases, calling the issue “a pretty urgent thing for us to get a handle on.” The agency is discussing a variety of possible epidemiological and clinical studies to examine post-Paxlovid rebound with scientists at the Centers for Disease Control and Prevention, he said.

The demand for answers is rising as Paxlovid has become a key element of the Biden administration’s pandemic approach, with the drug being made available at pharmacies nationwide. Among other questions the NIH hopes to get a better handle on is how often viral rebounds occur after five days of Paxlovid treatment, who’s at risk for relapse, and whether it could be avoided with a longer regimen.