After a brief period of COVID negativity, President Biden tested positive for COVID again over the weekend. However, there was no reemergence of symptoms and the "president continues to feel well," White House physician Dr. Kevin O'Connor declared, saying there is no need to reinitiate the therapy at this time. Due to the positive COVID test, Biden will also restart the CDC's recommended isolation procedure for at least five days.

Biden, who is fully vaccinated and received two boosters of the Pfizer-BioNTech (PFE, BNTX) vaccine, started a course of antiviral Paxlovid after first testing positive on July 21. He then tested negative for COVID from Tuesday to Friday, but was positive again on Saturday, according to the White House, which added that the case represents "rebound positivity." A similar event also happened to White House chief medical advisor Dr. Anthony Fauci, who tested positive for COVID (with returning symptoms) three days after finishing his Paxlovid regimen.

Commenting on the "rebound" linked to Paxlovid during its recent earnings call, Pfizer said last week that the company and the FDA are working together to design a study targeting patients who might need retreatment. Both Pfizer and the FDA pointed out that only 1%-2% of people in the original Paxlovid study saw their virus levels rebound after 10 days. The rate was also about the same among people taking the medicine or dummy pills, "so it is unclear at this point that this is related to drug treatment."

CDC: "Limited information currently available from case reports suggests that persons treated with Paxlovid who experience COVID-19 rebound have had mild illness; there are no reports of severe disease," the agency said in May. "Rebound" cases are said to resurface within two days to eight days after initially testing negative for the virus.