When Dr. Anthony Fauci contracted Covid in June, he took Paxlovid, an antiviral for people with mild to moderate symptoms whose age or immune status puts them at high risk of serious illness. The treatment consists of three pills taken twice a day for five days.
Fauci discontinued treatment and tested negative for Covid. But about three days later, a test came back positive. His symptoms – runny nose, sore throat and fever – also resurfaced.
A small minority of people taking Paxlovid see a similar rebound effect.
“If you look at the studies, it’s generally not very common,” Fauci, who heads the National Institute of Allergy and Infectious Diseases and is President Joe Biden’s chief medical adviser, said Tuesday on MSNBC’s “Morning Joe.” But he noted that there have been “anecdotal cases” suggesting that rebounds are now more common than in clinical trials.
About 1% to 2% of people taking Paxlovid in Pfizer’s clinical trial tested positive for the coronavirus after testing negative. Rebound rates are about 5% among the tens of thousands of people who have used the drug in real life, said White House Covid response coordinator Dr. Ashish Jha, at a press conference last week.
“When you look at Twitter, it feels like everyone has a rebound,” Jha said. “But it turns out that there is actually clinical data.”
A small study in June found that in less than 1% of Covid patients, symptoms returned on average about nine days after taking Paxlovid. In a larger study of 13,600 Covid patients, which was not peer-reviewed, 6% showed their symptoms in the month after treatment.
dr. Aditya Shah, an infectious disease specialist at the Mayo Clinic who led the small study, said it’s difficult for people to self-diagnose rebound cases. Ideally, people would have evidence that they tested positive, then negative, then positive again, he said.
The actual number of rebound cases “could be 5 to 10%, but I don’t think it’s as common as the general community makes out,” Shah said.
The Centers for Disease Control and Prevention said in May that symptoms usually return two to eight days after Paxlovid is stopped, if at all.
People who continue to test positive can still be contagious, so the CDC recommends that people restart isolation for at least five days if their disease returns.
The treatment may not last long enough
Some disease experts suspect that the Paxlovid regimen is too short to clear the virus in certain people.
“One of the theories is that your natural immunity just can’t kick in fast enough because you have to treat within five days of symptoms, so that’s pretty early in the course of the disease,” said Dr. Peter Gulick, an associate professor of medicine at Michigan State University.
When someone takes Paxlovid, “it probably knocks that viral load down to a point where the body doesn’t respond to it clinically with any kind of inflammation,” Gulick said. That would explain why the symptoms initially disappear in about five days.
But some people can still have a lingering virus in their bodies, especially if they had a high viral load to begin with or if the virus has spread to areas that drugs can’t easily reach. In that case, the symptoms may return.
“There may even be reservoirs,” Gulick said. “We don’t know Paxlovid’s ability to get into certain small areas of the body where the virus could be.”
However, Shah isn’t sure if extending the course of treatment would benefit patients: “I’d be hesitant to say now, ‘Yeah, sure, take a 10-day course,’ because there’s no evidence of that.”
Disease experts said it’s possible that older or immunocompromised people are more likely to get symptoms back or test positive again after taking Paxlovid, but there’s no good data yet to support the theory.
In any case, the rebounding symptoms should be mild. A June CDC survey found that less than 1% of people taking Paxlovid were admitted to the hospital or emergency department for Covid in the five to 15 days after they stopped treatment.
“Paxlovid works very well and prevents serious illness, rebound or no rebound,” Jha said last week. “That’s why the president took it.”
Biden completed his five-day course of Paxlovid on Monday. Experts said he should know if his symptoms have recovered in the coming days.