President Biden tested positive for a rebound case of COVID-19 on Saturday morning after being treated with Paxlovid, his physician said in a letter.

Driving the news: Biden is not experiencing new symptoms and “continues to feel quite well,” Dr. Kevin O’Connor said. “This being the case, there is no reason to reinitiate treatment at this time, but we will obviously continue close observation.”

  • Biden tested negative for COVID-19 on Tuesday, Wednesday, Thursday and Friday before the positive test Saturday.
  • The president will go into “strict isolation,” O’Connor said.

What he’s saying: “This happens with a small minority of folks,” Biden tweeted. “I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon.”

Catch up quick: The president’s case likely stemmed from the Omicron subvariant BA.5, currently the most dominant strain of the virus in the U.S., Axios’ TuAnh Dam writes.

  • Earlier this week, Biden completed a five-day course of Paxlovid, an antiviral treatment that helped resolve his COVID-19 symptoms. His runny nose and cough improved after the first full day of taking the pill.

The big picture: “Rebound” cases of COVID are possible for those who take Paxlovid, but it remains relatively rare, a Mayo Clinic study released in June found.

  • Pfizer’s chief executive told Bloomberg in May that patients who experience a rebound of symptoms after taking Paxlovid should resume the treatment.

Yes, but: The Food and Drug Administration said at the time there is “no evidence” the second round of treatment will help patients.

Editor’s note: This story has been updated with details throughout.