President Joe Biden experienced a "rebound" Covid infection after taking the antiviral drug Paxlovid — and he's not the only one.Some patients who took Pfizer's Paxlovid after contracting the coronavirus have reported the same phenomenon: Days after they finished a five-day course of the oral drug and felt better, their Covid symptoms or a positive test result returned.Health experts say Paxlovid's rebound effect doesn't impact every patient or make it any less effective at its job, which is fighting severe illness from Covid. Still, like with so much about the pandemic, you might have some questions: How severe are rebound cases? Why do they happen? How common are they, and should you still feel comfortable taking the drug?The answer to that last question is a resounding "yes," doctors say. Here's why, and what else you need to know about Paxlovid rebound cases:Who can take Paxlovid?In December 2021, the U.S. Food and Drug Administration made Paxlovid available under an emergency use authorization to treat mild-to-moderate Covid cases in a specific group of eligible patients. You can get Paxlovid if you check all three of these boxes:You tested positive for CovidYou're at least 18 years old, or at least 12 years old and weigh at least 88 poundsYou have one or more risk factors for severe CovidThat includes patients 65 and older — such as Biden, 79 — or those with underlying conditions like cancer, diabetes or obesity. You may not be able to take Paxlovid if you take certain medications that can interact with the drug and cause serious side effects, according to the FDA.You can obtain Paxlovid prescriptions from your healthcare provider or through the Biden administration's "Test to Treat" program, which gives free Covid antiviral pills to patients who test positive at pharmacies across the country.If you're eligible, you should start taking Paxlovid as soon as possible after testing positive for Covid, and within five days of experiencing Covid symptoms. You'll need to take three pills, twice a day, for five days.Pfizer's clinical trials last November suggest that Paxlovid does its job: The drug was 89% effective at preventing hospitalization among people who were at risk of developing severe illness.Notably, that trial was conducted before Covid's omicron variant emerged — but Pfizer said in January that Paxlovid still works against omicron, citing three laboratory-based studies. It appears to also work against omicron subvariants like BA.5, with no current data showing otherwise, according to Barbara Santevecchi, a clinical assistant professor of infectious diseases at the University of Florida's College of Pharmacy.How common are rebound cases, and what are they like?Some people who take Paxlovid test negative for Covid after finishing their five-day treatment, but then test positive or experience symptoms again two to eight days later, according to the Centers for Disease Control and Prevention.Roughly 5% of the tens of thousands of Paxlovid users have experienced rebound cases so far, White House Covid response coordinator Dr. Ashish Jha said at a news conference last month. They appear to be very mild: A June CDC study found that less than 1% of patients taking Paxlovid were admitted to the hospital or emergency department for Covid in the five to 15 days after they finished the treatment.Patients also appear to recover from rebound cases without any additional Covid treatment, the CDC says.A UC San Diego School of Medicine study released in June identified "insufficient drug exposure" as the most likely cause. In that scenario, Paxlovid stops the virus in its tracks for five days, but doesn't stick around long enough to purge the infection entirely — allowing the virus to temporarily replicate again once the drug is gone.Dr. Davey Smith, the study's senior author and an infectious disease specialist at UCSD Health, hypothesizes that some people may metabolize Paxlovid more quickly, or that the drug might need to be taken for more than five days to fully clear the virus in every patient. But there's no clinical data to back that up yet, he says."We don't know if it's safe or efficacious to do double the amount of time of Paxlovid, doing two courses," Smith tells CNBC Make It. "That's getting too far out over your skis without the clinical research to guide it."If you experience a rebound case, you do need to reenter quarantine until you test negative again. The CDC advises isolating for at least five more days before checking the agency's current isolation guidelines. You should also wear a mask for 10 days after rebound symptoms begin, the CDC advises.Should I still take Paxlovid if I'm eligible?
Biden's Covid relapse sparks talk of 'Paxlovid rebounds'—what to know about the pill, and if it could happen to you.