US President Joe Biden feels good and is looking forward to getting back on the road as he continues to test positive for Covid-19 and maintains isolation procedures, White House spokesperson Karine Jean-Pierre said on Monday.

"He's feeling fine," Jean-Pierre told reporters at a White House briefing.

Biden, 79, had just emerged from isolation on Wednesday after testing positive for Covid for the first time on July 21, when he tested positive again on Saturday in what his physician Kevin O'Connor described as a "rebound" case seen in a small percentage of patients who take the antiviral drug Paxlovid.

He tested positive again on Monday "as could be anticipated," O'Connor said in a memo released by the White House.

"He will continue to conduct the business of the American people from the Executive Residence," O'Connor wrote.

Jean-Pierre said Biden still has mild "lingering" symptoms from his first bout with Covid, but has not experienced any new symptoms, such as a fever.

Paxlovid is an antiviral medication from Pfizer that is used to treat high-risk patients, such as older patients.

Biden tweeted about his positive case on Saturday, saying it could happen to a "small minority of folks." He later posted a video on Twitter where he said he was "feeling fine" and "everything's good."

Biden's return to isolation came after he celebrated being able to work in person again at a Rose Garden event on Wednesday. (Reuters)