Paxlovid Doesn't Prevent Infection in Households, Pfizer Says

Carolyn Crist

May 02, 2022

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Paxlovid works as a treatment for COVID-19 but not as a preventive measure, particularly if you've been exposed to the coronavirus through a household member who is infected, according to a new announcement from Pfizer.

In a clinical trial, the oral antiviral tablets were tested for post-exposure prophylactic use, or tested for how well they prevented a coronavirus infection in people exposed to the virus. Paxlovid somewhat reduced the risk of infection, but the results weren't statistically significant.

"We designed the clinical development program for Paxlovid to be comprehensive and ambitious with the aim of being able to help combat COVID-19 in a very broad population of patients," Albert Bourla, PhD, Pfizer's chairman and CEO, said in the announcement.

"While we are disappointed in the outcome of this particular study, these results do not impact the strong efficacy and safety data we've observed in our earlier trial for the treatment of COVID-19 patients at high risk of developing severe illness," he said.

The trial included nearly 3,000 adults who were living with someone who recently tested positive for COVID-19 and had symptoms. The people in the trial, who tested negative and didn't have symptoms, were either given Paxlovid twice daily for 5 or 10 days or a placebo. The study recruitment began in September 2021 and was completed during the peak of the Omicron wave.

Those who took the 5-day course of Paxlovid were found to be 32% less likely to become infected than the placebo group. Those who took the 10-day treatment had a 37% risk reduction. But the results weren't statistically significant and may have been due to chance.

"Traditionally, it's been difficult to use small molecule antivirals for true prophylaxis because the biology of treating infection is different from the biology of preventing infection," Daniel Barouch, MD, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, told STAT News.

He also noted that the Omicron variant could have played a role.

"That hyper-infectiousness probably makes it more difficult to prevent infections," Barouch said.

The safety data was consistent with that of previous studies, Pfizer said, which found that the treatment was about 90% effective at preventing hospitalization or death in COVID-19 patients with a high risk of severe illness if the pills were taken for 5 days soon after symptoms started.

Paxlovid is approved or authorized for conditional or emergency use in more than 60 countries to treat high-risk COVID-19 patients, Pfizer said. In the U.S., the drug is authorized for emergency use for the treatment of mild to moderate COVID-19 in ages 12 and older who face high risks for severe disease, hospitalization, or death.

The full study data will be released in coming months and submitted to a peer-reviewed publication, the company said. More details are on the website (NCT05047601).


Pfizer: "Pfizer Shares Top-Line Results from Phase 2/3 EPIC-PEP Study of Paxlovid for Post-Exposure Prophylactic Use."

STAT News: "Paxlovid's failure as a preventative measure raises questions, but doctors still back it as a therapeutic." "NCT05047601: A Study of a Potential Oral Treatment to Prevent COVID-19 in Adults Who Are Exposed to Household Member(s) With a Confirmed Symptomatic COVID-19 Infection."


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