President Joe Biden tested negative for Covid-19 on Saturday, but “in an abundance of caution” will remain in strict isolation under daily monitoring until a second negative result confirms he has beaten his “rebound” infection, according to White House physician Dr. Kevin O'Connor.

“The President continues to feel very well,” O’Connor wrote in his daily update on Biden's condition.

The US leader initially tested positive for Covid-19 on July 21, despite having been fully vaccinated and receiving two booster shots. He worked in isolation for five days, while experiencing a mild cough and body aches, before testing negative last week.

However, just three days later, Biden tested positive once again, with O’Connor noting that “a small percentage of” patients being treated for Covid-19 with Pfizer’s antiviral pill Paxlovid are likely to suffer so-called “rebound Covid positivity” – where they seem to recover from the disease before falling ill again.

Following his first bout with the virus, Biden described Paxlovid as a “game-changer” in the fight against Covid-19, and urged Americans to obtain the pills from drugstores around the country.

“Limited information currently available from case reports suggests that persons treated with Paxlovid who experience COVID-19 rebound have had mild illness; there are no reports of severe disease,” the Centers for Disease Control and Prevention said in May. White House coronavirus czar Dr. Anthony Fauci also suffered such a reinfection when he treated his case of Covid-19 with Paxlovid.