April 29, 2022 -- U.S. researchers are planning to study how often and why coronavirus levels come back for some COVID-19 patients who take Pfizer’s Paxlovid drug, according to Bloomberg News.

The CDC and the National Institute of Allergy and Infectious Diseases are considering a variety of epidemiological and clinical studies to examine potential relapses after the five-day antiviral treatment, including which patients face higher risks and whether it can be avoided with a longer course of treatment.

“It is a priority,” Clifford Lane, MD, the institute’s deputy director for clinical research, told Bloomberg News, adding that the question is “a pretty urgent thing for us to get a handle on.”

Paxlovid treatment is becoming an increasing part of the Biden administration’s pandemic approach, the news outlet reported, and is more widely available at pharmacies nationwide.

Researchers know little about the rebound cases so far, including how often they occur, whether the Omicron variant may play a role, or whether the drug is even related to the relapse in the first place, the news outlet reported. Scientists are eager to learn more so that doctors know how to help patients who receive Paxlovid treatment and then experience a resurgence of infection.

Earlier this week, researchers at the VA Boston Healthcare System published a preprint with a case study, which marks the first rebound case described in medical literature. A 71-year-old man, who was vaccinated and boosted, quickly recovered from COVID-19 after taking Paxlovid but then had a relapse of symptoms and a spike in viral levels about nine days after his first positive test. Then the symptoms and viral levels resolved a few days later.

Similar cases have appeared on social media and in medical blogs, and doctors have seen patients with these types of rebounds in recent weeks.

“Providers who are going to be prescribing this should be aware that this phenomenon occurs, and if people have symptoms worsening after Paxlovid, it’s probably still COVID,” Paul Sax, MD, clinical director of the infectious diseases division at Brigham and Women’s Hospital, told Bloomberg News.

Sax wrote about a post-Paxlovid rebound he saw a few weeks ago in one of his HIV patients.

Pfizer’s final-stage trial showed that a small number of patients experienced a rebound after being treated. The FDA previously said it was “evaluating the reports of viral load rebound after completing Paxlovid treatment and will share recommendations if appropriate.”

“The big problem is that when this drug was released, this information wasn’t included” in the label, Sax said.

The upcoming CDC and National Institutes of Health studies will attempt to understand how and why the rebound happens. If it appears to be related to emerging variants such as Omicron or certain patient risk factors, some people may need longer than the standard five-day treatment.

“There’s two things that suppress the virus: the drug and the host immune response,” Lane said. “If you stop the drug before the host immune response has had a chance to kick in, you may see the virus come back.”

Show Sources

Bloomberg News: “U.S. Seeks ‘Urgent’ Data on Covid Relapses After Using Pfizer’s Drug.”

ResearchSquare: “Rapid Relapse of Symptomatic SARS-CoV-2 Infection Following Early Suppression with Nirmatrelvir/Ritonavir.”

NEJM Journal Watch: “Yes, Relapses After Paxlovid Happen — Now What?”

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