President Joe Biden is once again testing positive for COVID-19 in what White House doctors are calling a “rebound” case that happens in a rare number of people treated with Paxlovid.

The antiviral pill developed by Pfizer is the first pill approved by the Food and Drug Administration for treatment of COVID-19. It was granted emergency use authorization in December.

“After testing negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning, the President tested positive late Saturday morning, by antigen testing,” White House doctor Kevin O’Connor said Saturday in a news release.

Biden, who initially tested positive on July 21, is among a small percentage of Paxlovid users to experience what has become known as the “Paxlovid rebound.” Users will complete a round of treatment, begin testing negative for the coronavirus, and then suddenly begin testing positive again.

Dr. Anthony Fauci, the nation’s top infectious disease expert, took Paxlovid after testing positive for COVID. He also said he had a recurrence of symptoms after completing a full round of treatment.

The antiviral medication has proven very effective, but data from Pfizer now shows the rebound is happening in about 10% of patients.

O’Connor said there is no need for the president to begin another round of treatment; rather, he will return to strict isolation while he waits to again test negative for the virus.

Biden said on Twitter that he was feeling fine, did not have any symptoms and was continuing to work, albeit with quarantine precautions.

“Folks, today I tested positive for COVID again,” the president wrote on Twitter. “This happens with a small minority of folks. 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.”

Paxlovid isn’t approved to prevent a person from getting COVID, or in place of getting a vaccine. And despite concerns about the Paxlovid rebound, the medication is still in high demand.