President Joe Biden tested positive for Covid again late Saturday morning in a "rebound" case after testing negative several days in a row, his doctor said in a letter.
"After testing negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning, the President tested positive late Saturday morning, by antigen testing. This in fact represents 'rebound' positivity," wrote White House physician Dr. Kevin O’Connor.
Biden, 79, has not experienced any re-emergence of symptoms and continues to feel well, O'Connor said. "This being the case, there is no reason to reinitiate treatment at this time, but we will obviously continue close observation."
He will, however, return to strict isolation procedures, the doctor added.
A White House official said they are in the process of contact tracing to determine close contacts.
Vice President Kamala Harris is not considered a close contact, and tested negative for Covid-19 on Friday, according to her press secretary, Kirsten Allen.
Biden, who is fully vaccinated and twice boosted, tested positive for Covid on July 21 during a routine test, and was treated with the antiviral Paxlovid.
A small minority of people who take Paxlovid see a rebound effect. Around 1% to 2% of people taking Paxlovid in Pfizer’s clinical trial tested positive for the coronavirus after having tested negative. Rebound rates are around 5% among the tens of thousands of people who’ve taken the drug in real-life settings, White House Covid response coordinator, Dr. Ashish Jha, said at a recent news conference.
The Centers for Disease Control and Prevention warned of the possible recurrences in May, and said that people who test positive again may still be contagious and should restart isolation for at least five days.
"Folks, today I tested positive for COVID again. This happens with a small minority of folks," the president tweeted Saturday. "I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon."
He left isolation Wednesday after five days. At the time, O'Connor said his symptoms had almost completely resolved.
Biden on Thursday delivered remarks in the State Dining Room and participated in a roundtable with officials as well as some CEOs. He did not wear a face mask at either event.
A White House official said Saturday that Biden would wear a mask around others through Day 10, in alignment with CDC guidance.
"When he was at a podium or similar circumstance we ensured that there is sufficient distance between him and others to allow him to safely remove his mask," the official said. "He was at a safe distance from others at those events.”
Dr. Anthony Fauci, chief medical adviser to Biden, who had Covid in June, also said he went through Paxlovid rebound.