People who recover with COVID-19 after Paxlovid may be highly contagious, new studies suggest

The CDC warning cited recent reports from researchers who documented some Paxlovid rebounds, even among patients who were vaccinated and boosted.

The CDC warning comes as two small but provocative new studies from different teams of researchers in Boston and New York suggest such relapses may not be that unusual. 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 family members. In one case, a 67-year-old man with no symptoms six days after completing Paxlovid infected a 6-month-old relative. The broadcast took place after the CDC-suggested window of time for isolation.

“This is not uncommon, otherwise why are we seeing these clusters?” said Dr. Michael Charness, chief of staff at the VA Boston Healthcare System and co-senior author of the study, which looked at COVID infections in 10 patients, ages 31 to 71, who were fully vaccinated and had received at least one vaccine. reinforcement. The study was published online Monday and has not been peer-reviewed.

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

When Paxlovid’s maker, Pfizer, received emergency use authorization for the drug in December from federal regulators, its data indicated that about 2 percent of patients in its trial experienced recovery, as did about 1.5 percent of those who received a placebo.

The Charness study doesn’t directly refute the Pfizer data, but it does suggest that rebounds are more common. Patients in that study experienced relapse symptoms beginning three to eight days after completing Paxlovid, and symptoms lasting three to ten days.

Antigen tests performed while patients were relapsed indicated that they remained positive for a median of six days, and up to day 18 after their initial positive pre-Paxlovid test.

For comparison, Charness’s group looked at a separate group of COVID patients who had not taken Paxlovid: nearly 1,000 National Basketball Association staffers. None had a COVID relapse. These data have not been published.

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

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

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

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

A second study by a team of researchers in Boston looked at seven patients who 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 for 11 days after stopping Paxlovid.

“This greatly increased the suspicion that they are contagious,” said Dr. Mark Siedner, an infectious disease physician and researcher at Massachusetts General Hospital and co-author of the study, which has not been peer-reviewed.

“It’s not just that they rebound. It is that they recover and it seems that they start from the beginning, their virus goes up a lot,” 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 another drug.

The CDC alert doesn’t definitively answer that question.

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

Rebounds with such high levels of live virus have led Siedner to wonder if there is something in Paxlovid that could be contributing to the phenomenon.

“It makes us wonder if we’re not using the drug correctly or for long enough, or is it something inherent in Paxlovid that doesn’t allow the immune system to kick in?” he said.

Siedner’s team, which includes researchers from Brigham and Women’s Hospital, as well as the Broad Institute and the Ragon Institute at 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 response of those who received Paxlovid is different from that of those who did not receive Paxlovid.

Dr. Kathryn Stephenson, an assistant professor at Harvard Medical School and an infectious disease physician at Beth Israel Deaconess Medical Center, is also conducting a Paxlovid studymonitoring patients just started on the antiviral with COVID tests for two to three weeks for rebound information on symptoms.

He said his 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 produce and share this data quickly, it’s their drug [that received emergency use authorization,]Stephenson said.

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

Kay Lazar can be reached at Follow her on Twitter @GlobeKayLazar.

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