Some coronavirus-positive patients who have completed treatment with the anti-COVID drug Paxlovid are making a recovery, and experts are urging people to be cautious if they develop COVID-like symptoms again and become contagious.

It’s unclear how often a “post-Paxlovid rebound” occurs, but Dr. Robert Wachter, chair of the medical school at UC San Francisco, said he knows at least one person who completed Paxlovid treatment and then became infectious again and passed the virus on to other family members.

“It can happen,” Wachter tweeted. “If you develop recurring symptoms and a [positive] Rapid test, you are contagious. Please act accordingly.”

Los Angeles County Director of Public Health Barbara Ferrer said COVID-19 relapses after Paxlovid were “real.”

“They happened in large enough numbers that they were noticed by many people in many different places,” she said.

In a statement on its website, the U.S. Food and Drug Administration said it was aware of reports of COVID-19 symptoms returning after stopping treatment with Paxlovid. “In some of these cases, patients have tested negative on a direct SARS-CoV-2 virus test and then tested positive again,” the FDA said.

The agency said there were some patients in the Paxlovid clinical trial — around 1% to 2% — who tested negative and then turned positive. The finding wasn’t just in people taking Paxlovid; it also occurred in those taking the placebo.

“But judging by all the anecdotes, rebound seems to be more common – we’re waiting for good data,” said Wachter tweeted. Wachter suggested that a person who completes a Paxlovid course and then tests positive again should be considered contagious.

Paxlovid’s clinical trial data was collected when the The delta variant of the coronavirus dominated nationwide before the emergence of the far more transmissible Omicron family that is now circulating.

“The question is, ‘Is that more common with Omicron?’ We are currently working very hard to clarify this. We’re talking to healthcare systems and getting real-world experience data,” said Dr. Ashish Jha, the White House COVID-19 response coordinator.

But even if the rebound rate has stayed the same, the number of people affected could now appear much larger due to the drug’s broader use.

“If you have 20,000 people getting Paxlovid every day…even if it was down to 2%, that would mean 400 people have a rebound every day. So it’s hard to know how often it happens,” Jha said during a briefing on Wednesday.

However, he also noted that “the people who have that rebound don’t get particularly sick and they don’t end up in the hospital.”

“If the goal of this treatment was to prevent hospitalizations and deaths, it does that incredibly well,” he said.

Those who want to be extra vigilant about the risk of contaging others should consider wearing a mask in all indoor spaces, including around family members in the same household, e.g a few days more or maybe a week once Paxlovid treatment is complete, Wachter said.

and dr Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said her agency would “recommend putting the mask back on” should symptoms return.

Even a negative rapid test after completing five days of Paxlovid treatment can give false assurance that a person is not contagious. A negative rapid test result suggests a person is unlikely to be contagious, but there’s still a small chance — maybe 1% — that a person can test negative and still be contagious, Wachter wrote.

“Wearing masks for a few days seems reasonable if you’re careful,” he said.

The COVID-19 rebound after Paxlovid still seems to be unusual, said Wachter, and is not harmful to the convalescent patient. Wachter said news about the recovery shouldn’t put people off taking Paxlovid because of its high potency against serious illness and death.

Data has shown that Paxlovid, made by Pfizer, reduces the risk of hospitalization or death from COVID-19 by 89% in higher-risk, unhospitalized adults.

Additionally, it’s plausible but unproven that “quickly lowering viral loads (which Paxlovid does) could reduce the risk of long-lasting COVID and make people less contagious,” Wachter said wrote.

The FDA agreed that the reports of the COVID-19 rebound after Paxlovid do not change scientists’ overall view of the drug’s high efficacy.

“Most importantly, there was no increased incidence of hospitalizations or deaths, or development of drug resistance,” the FDA said. “So these reports do not change the conclusions from the Paxlovid clinical trial, which showed a significant reduction in hospital admissions and deaths.”

The FDA said patients should wear a mask and stay home and isolate if they have symptoms of COVID-19, regardless of whether they’ve been given an antiviral like Paxlovid.

dr Anthony Fauci, the nation’s top infectious disease expert, said Wednesday that the National Institutes of Health also plans to investigate whether prolonged treatment with Paxlovid is needed.

A report by UC San Diego scientists, posted online Wednesday, analyzed the case history of a traveler who returned to the United States after a trip to South Africa and developed recurrent COVID-19 after completing a Paxlovid course. The scientists said the most likely possibility for the recurring symptoms was “inadequate drug exposure.”

Among those watching the rebound was Dr. Paul Sax, an infectious disease expert at Harvard Medical School, who recently wrote a blog post about how a patient with COVID-19 recovered quickly after taking Paxlovid, but called him back a week later and said she had relapsed. Though the symptoms weren’t as bad as her original illness, quick test results showed she was “clearly positive again.”

“Her biggest concern was getting out into the world again without infecting anyone. She really wasn’t that sick; She just wanted advice on when to go back to work and socialize again,” Sax wrote. “‘Avoid close contact with others until this test is over,’ I said.”

The patient made a full recovery and she began testing negative a few days later, Sax wrote.