United States President Joe Biden on Saturday tested positive for the coronavirus disease, three days after he was allowed to come out of isolation.

The White House said it was due to a rare case of “rebound” seen in a small percentage of patients who take the antiviral drug Paxlovid during their treatment of Covid-19. Biden had tested positive for the coronavirus on July 21.

Paxlovid is being used in the United States for treating mild to moderate coronavirus in persons aged 12 years and older who are at high risk for severe disease.

White House physician Dr Kevin O’Connor said that Biden has experienced no re-emergence of symptoms and continues to feel quite well.

“This being the case, there is no reason to re-initiate treatment at this time, but we will obviously continue close observation,” Connor said. “However, given his positive antigen test, he will re-initiate strict isolation procedures.”

In a tweet, President Biden said that he was in isolation for the safety of everyone around him.

“I’m still at work, and will be back on the road soon,” he wrote.

Biden is at increased risk for severe disease as he is 79 years old. He had received his first two doses of Pfizer’s Covid-19 vaccine before taking over as president. He got his first booster shot on September 27 and an additional booster dose on March 30.