Biden had recovered last week from a first bout of COVID, but after four days of negative tests he once again tested positive Saturday.
"The president continues to feel well as he starts his week," White House physician Kevin O'Connor wrote in a memorandum on Monday.
"This morning, as could be anticipated, his SARS-CoV-2 antigen testing remained positive. The president will continue his strict isolation measures," the doctor wrote. "He will continue to conduct the business of the American people."
In a memorandum Sunday, O'Connor linked that result to "rebound positivity."
He said patients treated with Paxlovid -- as Biden was -- sometimes clear the virus but still test positive afterward.