US President Joe Biden tested positive for Covid-19 for a second time and is returning to isolation though he "continues to feel quite well," his White House doctor said Saturday.

Biden "tested positive late Saturday morning, by antigen testing," and "will reinitiate strict isolation procedures," presidential physician Kevin O'Connor wrote in a memorandum.

"This in fact represents 'rebound' positivity," O'Connor wrote, referring to a situation in which patients treated with the drug Paxlovid -- as Biden was -- clear the virus but test positive after completing their course.