Biden, 79, who emerged from COVID isolation on Wednesday after testing positive on July 21, said he was feeling fine.

He would return to strict isolation and would cancel planned trips to his home in Wilmington and work trip in Michigan, the White House said.

Biden held public events on Wednesday and Thursday, but none on Friday.

The forced isolation came as the White House was hoping to celebrate some recent legislative victories to help boost Biden's slumping poll ratings.

Biden had planned the Michigan trip to tout Thursday's passage of legislation to boost the US semiconductor chips industry.

Biden's positive test is believed to be a "rebound" experienced by some COVID-19 patients who take the anti-viral drug Paxlovid, according to White House physician Kevin O'Connor.

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

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.

White House officials had previously suggested a rebound case of COVID was unlikely, based on reports of cases around the country.

However, Biden continued to be tested and monitored.

Biden tweeted about his positive case, saying it can 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".

A White House official said contact tracing efforts were under way on Saturday after Biden's positive COVID-19 test.

National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci also experienced rebound COVID-19.

His symptoms got worse when they returned after treatment, and his doctors prescribed another course of Paxlovid.

O'Connor said Biden tested negative for the past four days, and there was no plan to re-initiate treatment given his lack of symptoms.

Australian Associated Press