U.S. President Joe Biden is feeling well and continuing his isolation measures after again testing positive for COVID-19, his physician said in a memo released by the White House on Sunday.

Biden tested positive for COVID again on Saturday in what the White House doctor described as a “rebound” case seen in a small percentage of patients who take the antiviral drug Paxlovid.

“Given his rebound positivity which we reported yesterday, we continued daily monitoring. This morning, unsurprisingly, his SARS-CoV-2 antigen testing remained positive,” the physician, Dr. Kevin O’Connor, said in the memo on Sunday.

Biden tested positive for COVID for the first time on July 21 and previously described his experience with the coronavirus as mild, saying he was able to continue working while in isolation and attributed his relative ease with the disease to vaccines and other treatments.

Biden, 79, had initially emerged from COVID isolation on Wednesday when he tested negative.

After testing positive again, the president had to cancel some trips and events to celebrate recent legislative victories and to help boost his slumping poll ratings.

A small but significant percentage of people who take Paxlovid will suffer a relapse or a rebound that occurs days after the five-day treatment course has ended, studies have shown.

“Biden will continue to conduct the business of the American people from the Executive Residence,” his physician said on Sunday.

The White House had sought to underscore Biden’s ability to work through his illness when he first tested positive, releasing videos of him assuring Americans he was doing fine and of his participation in virtual meetings with White House staff.