She is isolating at home and will participate in meetings virtually, the agency added.

Walensky joins a short list of prominent U.S. pandemic figures, including President Joe Biden and presidential physician Dr. Anthony Fauci, who’ve tested negative, then positive again after taking the popular drug, prescribed for those with documented COVID who are at high risk for severe disease.

Just how rare are rebounds?

Paxlovid, approved for use by the U.S. Food and Drug Administration under an emergency use authorization, has become known for its “rebound cases,” referred to by the CDC as a “brief return of symptoms.”

Such rebounds, however, can happen with or without Paxlovid, Andy Pekosz, virologist at the Johns Hopkins Bloomberg School of Public Health, recently told Fortune.

While the U.S. Centers for Disease Control and Prevention says that COVID rebounds are rare, they could be more common than we know, as most individuals with COVID stop testing after they receive a negative result, he added.

“After I finished the five days of Paxlovid, I reverted to negative on an antigen test for three days in a row,” Fauci, 81, said during an event at Foreign Policy’s Global Health Forum, Bloomberg reported. “And then on the fourth day, just to be absolutely certain, I tested myself again.”

“I reverted back to positive.”

Fauci began a second course of Paxlovid when symptoms emerged “much worse than the first go-around,” he said. In May the CDC issued a health advisory about such rebounds, saying there was no evidence that additional treatment is needed for rebound cases.

In June, Pfizer, which manufactures Paxlovid, announced that it would stop adding new participants to a trial of the drug among COVID patients at low risk of hospitalization and death. The study failed to demonstrate that the drug reduced symptoms, or hospitalizations and deaths, in a statistically significant way, according to Bloomberg.

But Paxlovid may be undeservedly gaining a bad rap, Pekosz cautioned.

“I’ll still point to the fact that it’s working in terms of keeping people out of the hospital—that’s the most important thing right now,” he said, adding that it may need to be retooled to better address Omicron subvariants.