(NEXSTAR) — Pres. Joe Biden has tested positive for COVID-19 again, just over three days after he was cleared to exit isolation, the White House announced Saturday. The positive test represents a “rebound” case after treatment with the anti-viral drug Paxlovid, WH physician Dr. Kevin O’Connor said in a letter.

O’Connor said Biden, who initially tested positive on July 21, “has experienced no reemergence of symptoms and continues to feel quite well.”

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. Though it has been approved, data from Pfizer shows some users can experience a rebound after completion of a round of treatment. Users will test negative 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.

Dr. Bud Lawrence, medical director at Henry Mayo Newhall Hospital’s ER department, told Nexstar affiliate KTLA in Los Angeles Paxlovid rebound is “definitely a real thing.”

“It is something that is occurring with some patients,” Lawrence told KTLA. “They take the Paxlovid and anywhere from 10 to 14 days from the initial onset of symptoms, they get another bump in symptoms.”

So how common is ‘Paxlovid rebound’?

Data from Pfizer now shows the rebound is happening in about 10% of patients, which is relatively small.

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.”

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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.