(NewsNation) — President Joe Biden tested positive for COVID-19 again on Sunday in a rare case of “rebound” following treatment, the White House physician said in a note.

The president initially tested positive after several negative tests on Saturday.

Dr. Kevin O’Connor wrote that Biden, 79, who was treated with the anti-viral drug Paxlovid, has experienced no reemergence of symptoms and will isolate following the positive test.

 “[Biden] has experienced no reemergence of symptoms, and continues to feel quite well,” O’Connor wrote.

A rebound case like the one Biden has is “extrememly uncommon,” Dr. Anthony Harris, CEO of Hfit Health told NewsNation.

“It’s about 1% of the population treated with Paxlovid that experience a rebound,” Harris said. “The good news is that we shouldn’t be seeing this occur in the general population, again, no more than 1%.”

In accordance with Centers for Disease Control and Prevention guidelines, Biden will reenter isolation for at least five days.

“I’ve got no symptoms but I am going to isolate for the safety of everyone around me,” Biden said on Twitter. “I’m still at work, and will be back on the road soon.”

The agency says most rebound cases remain mild and that severe disease during that period has not been reported.

“I’m feeling fine, everything is good,” Biden said in a short video posted on social media, a pair of aviator sunglasses in his hand. “But Commander and I got a little work to do.”

Harris said there is no evidence this was failed treatment of Biden and that Paxlovid should still be considered a highly effective COVID-19 treatment drug.

Biden’s positive result by antigen testing follows several negative tests on Tuesday evening, Wednesday morning, Thursday morning and Friday morning, according to the White House.

On Tuesday, Biden had improved enough from his coronavirus infection that he resumed his regular exercise routine, according to an update from his doctor.

His positive tests puts him among the minority of those prescribed Paxlovid to experience a rebound case of the virus.

White House COVID-19 coordinator Dr. Ashish Jha told reporters on July 25 that data “suggests that between 5 and 8 percent of people have rebound” after Paxlovid treatment.

Word of Biden’s positive test came just two hours after the White House announced a presidential visit to Michigan this coming Tuesday.

Biden had also been scheduled to visit his home in Wilmington, Delaware, on Sunday morning, where first lady Jill Biden has been staying while the president was positive.

Both trips have been canceled. According to the press office, he’ll remain at the White House until he tests negative again.

The Associated Press contributed to this report.