WASHINGTON - US President Joe Biden remained positive for COVID-19 on Sunday but "continues to feel well," his physician said.

White House physician Dr. Kevin O'Connor wrote in a memo that "this morning, unsurprisingly, his SARS-CoV-2 antigen testing remained positive."

"The President will continue his strict isolation measures as previously described," O'Connor's memo read.

Biden tested positive for COVID-19 on Saturday in a "rebound" case, days after ending isolation and treatment.

"I've got no symptoms but I am going to isolate for the safety of everyone around me," he tweeted. "I'm still at work, and will be back on the road soon."

Biden, 79, first tested positive for the coronavirus on July 21 before receiving negative testing results several days later.

Fully vaccinated and twice boosted, Biden took Paxlovid, an antiviral therapy produced by Pfizer and given to patients with COVID-19.

There is potential for so-called "rebound" COVID positivity observed in a small percentage of patients treated with Paxlovid, according to O'Connor.