Patients who experience recurrence of COVID-19 symptoms after completing treatment with Pfizer's drug Paxlovid should isolate again for five days, the U.S. Centers for Disease Control and Prevention said in an advisory issued on Tuesday.

Dozens of individuals have reported rebounding COVID symptoms on social media or to the U.S. Food and Drug Administration after taking Paxlovid, but Pfizer suggests the experience is rare. A recent rise in COVID cases has driven up use of therapeutics in the country.

The CDC said in its advisory that case reports suggest that recurrence results in mild symptoms, and there have been no reports of severe disease so far. The agency still continues to recommend the oral antiviral drug as a treatment, despite the possibility of recurrences.

A brief return of symptoms may be part of the natural history of SARS-CoV-2 infection in some persons, regardless of treatment with Paxlovid, the agency said.

CDC recommends that patients with COVID rebound can end their re-isolation period after five full days if there is no fever for 24 hours and symptoms improve, but such patients should wear a mask for a total of 10 days after rebound symptoms begin.