People recovering from COVID-19 after Paxlovid could be highly contagious, new studies suggest

The CDC warning cited recent reports from researchers documenting some Paxlovid rebounds, including in patients who have been vaccinated and boosted.

The CDC warning comes because two small but provocative new studies from different teams of researchers in Boston and New York suggest such relapses may not be all that uncommon. The scientists also found that in some rebound patients, viral levels were likely high enough to be contagious.

In addition, one study found that two patients who inadvertently relapsed infected relatives. In one case, a 67-year-old man without symptoms infected a 6-month-old family member six days after completing Paxlovid. The transmission took place after the time window for isolation proposed by the CDC.

“This is not rare. Why else do we see these clusters?” said Dr. Michael Charness, chief of staff for the VA Boston Healthcare System and co-lead author of the study, which analyzed COVID infections in 10 patients, ages 31 to 71, who had been fully vaccinated and given at least one booster shot. The study was posted online on Monday and has not been peer-reviewed.

Paxlovid is a home treatment prescribed at the first sign of infection to patients at high risk for serious COVID complications. The treatment consists of a total of 30 pills – three pills taken twice a day for five days.

When Paxlovid manufacturer Pfizer received emergency use of the drug from federal regulators in December, the data indicated that about 2 percent of patients in the study experienced a rebound, as did about 1.5 percent of those taking a received placebo.

The Charness study doesn’t directly refute the Pfizer data, but it does suggest that rebounds are more common. Patients in that study had symptoms of relapse that started between three and eight days after completing Paxlovid, and the symptoms lasted between three and 10 days.

Antigen tests performed while the patients relapsed indicated that they remained positive for an average of six days, and until day 18 after their first, pre-Paxlovid positive test.

For comparison, the Charness group looked at a separate group of COVID patients who had not taken Paxlovid: nearly 1,000 National Basketball Association staff. None of them had a relapse from COVID. They have not published that information.

“We quote the NBA data to say that this is clearly different from what was observed” by Pfizer, said Dr. David Ho, director of the Aaron Diamond AIDS Research Center at Columbia University and a co-author of the Charness and NBA studies.

A Pfizer spokesperson said the company continues to monitor data from its ongoing investigations into Paxlovid, as well as reports from doctors and patients of rebounds. He said all data is “consistent with our observations” from the company’s drug trial.

The new CDC warning notes that a COVID-19 relapse with a brief return of symptoms “may be part of the natural history of infection with SARS-CoV-2 (the virus that causes COVID-19) in some individuals), independently treatment with Paxlovid and regardless of vaccination status.”

Federal data shows that more than 668,000 courses of Paxlovid were prescribed as of May 14. Doctors say that despite the rebounds, it is an effective drug that has kept at-risk patients out of the hospital.

A second study by a team of Boston researchers analyzed seven patients who had recovered after taking Paxlovid and found levels of live virus in three of them for up to nine days. One of the patient’s test samples showed live virus 11 days after stopping Paxlovid.

“This greatly increased the suspicion that they are contagious,” says Dr. Mark Siedner, an infectious disease physician and researcher at Massachusetts General Hospital and co-author of the non-peer-reviewed study.

“It’s not just that they bounce back. It’s that they bounce back and it seems like they start from scratch, their virus goes way up,” Siedner said. “It’s a really unique phenomenon.”

Siedner’s team found no evidence that the virus developed resistance to Paxlovid.

Siedner and other researchers say the rebound phenomenon raises pressing questions about whether patients should be treated with a longer course of Paxlovid or perhaps with a different drug.

The CDC warning does not provide a definitive answer to that question.

“There is currently no evidence that additional treatment with Paxlovid or other anti-SARS-CoV-2 therapies is needed in cases where a rebound from COVID-19 is suspected,” it said.

The rebounds with such high levels of live virus have prompted Siedner to wonder if: there is something about Paxlovid that could contribute to the phenomenon.

“It makes us wonder, are we not taking the drug well or long enough, or is this something inherent in Paxlovid that is preventing the immune system from kicking in?” he said.

Siedner’s team, which includes researchers from Brigham and Women’s Hospital, as well as the Broad Institute and Ragon Institute of MGH, MIT and Harvard, is launching a new study that hopes to answer some of these questions.

They will test the immune systems of people who have recovered to see if the immune responses of those who have received Paxlovid are different from those who have not.

dr. Kathryn Stephenson, an assistant professor at Harvard Medical School and an infectious disease physician at Beth Israel Deaconess Medical Center, also leads a Paxlovid studymonitoring patients just starting the antiviral with COVID testing for two to three weeks to detect rebound information about symptoms.

She said her and other small studies of a few dozen people are helpful, but much larger, more rigorous studies are urgently needed to understand and address rebounds.

“I think it’s Pfizer’s responsibility to quickly produce and share this data – it’s their medicine” [that received emergency use authorization,]’ said Stephenson.

“It’s not fair that individual clinicians and researchers are now trying to catch up and collect this data on their own.”


Kay Lazar can be reached at [email protected] Follow her on Twitter @GlobeKayLazar

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