WASHINGTON, United States — Joe Biden tested positive for Covid-19 on Sunday for a second straight day but “continues to feel well,” the US president’s physician said.

US President Joe Biden speaks virtually during a meeting with CEOs and labor leaders, in the South Court Auditorium of the Eisenhower Executive Office Building, next to the White House, in Washington, DC. Brendan SMIALOWSKI / AFP

Biden had been recovering early in the week from a first bout of Covid, but after four days of negative tests again received a positive result on Saturday.

In a memorandum Sunday, White House doctor Kevin 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.

“The president will continue his strict isolation measures,” O’Connor said, but “he will continue to conduct the business of the American people from the executive residence.”

O’Connor said Saturday that Biden had “experienced no re-emergence of symptoms,” meaning no further treatment was required.

Although Biden, who is fully vaccinated, had to cancel planned trips to Delaware and Michigan, he tweeted Saturday that “I’m still at work, and will be back on the road soon.”

Biden, who will turn 80 in November, is the oldest US president in history, but O’Connor has declared him to be generally in good health.