(NewsNation) — President Joe Biden has tested negative for COVID-19 following treatment of a rare “rebound” case, the White House physician said Saturday.

“The President continues to feel very well,” Dr. Kevin C. O’Connor said in a letter to the president but noted that Biden, 79, will continue to isolate until he gets a second negative test.

Biden, who is fully vaccinated and twice boosted, first tested positive for COVID-19 in July, with mild symptoms. He was treated with the anti-viral drug Paxlovid.

He tested positive for the coronavirus again later that month, in a rebound case that is “extremely uncommon,” according Dr. Anthony Harris, CEO of Hfit Health.

“It’s about 1% of the population treated with Paxlovid that experience a rebound,” Harris told NewsNation “The good news is that we shouldn’t be seeing this occur in the general population.”

White House COVID-19 coordinator Dr. Ashish Jha said data “suggests that between 5 and 8% of people have rebound” after Paxlovid treatment.

The Associated Press contributed to this report.