(NewsNation) — First lady Jill Biden tested positive Wednesday for a rebound case of COVID-19, after testing negative for the virus earlier this week.

Characterized by the Centers for Disease Control and Prevention as a recurrence of symptoms or new positive viral test after testing negative, some people experience a “rebound case” even after taking the antiviral pill Paxlovid, as Biden did.

Paxlovid, developed by Pfizer, is the first pill approved by the Food and Drug Administration for treatment of COVID-19.

According to NBC, White House COVID response coordinator Dr. Ashish Jha said rebound rates make up around 5% of the tens of thousands of people who’ve taken the drug.

Dr. Aditya Shah, an infectious disease specialist at the Mayo study, told NBC it’s difficult for people to self-diagnose, adding that the real number of rebounds could be as high as 5% to 10% of people getting it.

“I don’t think it is as common as the general community is making it out to be,” Shah said.

Although experts say it is rare, several notable people have reported having rebound cases.

Jill Biden

The first lady, 71, is twice-vaccinated and double-boosted. After initially testing positive for COVID-19 on Aug. 16. she was prescribed the anti-viral drug Paxlovid, and isolated for five days. Although she tested negative for coronavirus Sunday, she tested positive again a few days later.

Biden, who is currently isolating in Deleware, has experienced no reemergence of symptoms with her “rebound” case.

Meanwhile, her husband, Joe Biden, tested negative for COVID on Wednesday morning on an antigen test. He will still mask for 10 days when indoors and in close proximity to others, however, the White House said.

Joe Biden

The president dealt with his own rebound case in late July. Like Jill Biden, he also didn’t see any reemergence of symptoms, but still isolated.

“I’m feeling fine, everything is good,” Biden said in a short video posted on social media, at the time. “But Commander and I got a little work to do.”

Anthony Fauci

Dr. Anthony Fauci, Biden’s chief medical adviser, had a rebound of COVID-19 symptoms after taking Paxlovid.

He initially tested positive for the virus June 15, and started getting symptoms again later that month. The symptoms Fauci had during his rebound case, he said, were worse than his first one, and included a low fever, achiness, runny nose and mild cough, according to the New York Times.

Fauci attributed the fact that he didn’t need to be hospitalized to the use of Paxlovid.

“I think there is understandable confusion when people hear about people rebounding,” Fauci told the New York Times. “Don’t confuse that with the original purpose of what Paxlovid is meant for. It’s not meant to prevent you from rebounding. It’s meant to prevent you from being hospitalized. I’m 81 years old, I was at risk for hospitalization and I didn’t even come close to being sick enough to be hospitalized.”

Stephen Colbert

The comedian and talk show host had to cancel the taping of his show in May because of a “recurrence” of COVID, the LA Times reported

The return of Colbert’s COVID symptoms came exactly a week after he returned to “The Late Show” after his first bout of coronavirus.

Colbert, in reaction to the news, tweeted: “WORST. SEQUEL. EVER.”

After Biden announced his own rebound case, Colbert joked, “Wow, getting Covid twice in a row ’cause you took Paxlovid? Who could’ve seen this coming? It happened to me,” the New York Times said.