An experimental COVID-19 pill developed by Japan¡¯s Shionogi cut the risk of infection among people exposed to the disease at home, according to a study that could revive interest in antivirals as a preventive tool.
In a late-stage trial of more than 2,000 household contacts, 2.9% of those given Shionogi¡¯s once-daily pill ensitrelvir developed symptomatic COVID-19 within 10 days, compared with 9.0% on placebo ¡ª a roughly two-thirds reduction in risk.
The study, published Wednesday in The New England Journal of Medicine, tested a five-day course started within 72 hours after a household member fell ill. Participants were initially virus-negative and most had some prior immunity through vaccination or earlier infection. No COVID-related hospitalizations or deaths were reported, and overall side effects were similar between groups.
The results mark the first late-stage success for an oral antiviral used to prevent COVID-19 after exposure ¡ª an approach that hasn¡¯t shown clear benefit in earlier trials of Pfizer¡¯s Paxlovid and Merck¡¯s molnupiravir.
That¡¯s likely because treatment in those studies began later ¡ª up to four or five days after exposure ¡ª reducing its effectiveness, said Frederick Hayden, the study¡¯s lead author and a professor emeritus of clinical virology at the University of Virginia School of Medicine, in an interview.
COVID-19 continues to cause substantial illness, with U.S. hospitalizations estimated at between about 120,000 and 250,000 and deaths ranging from roughly 12,000 to 41,000 from October 2025 through May 2, according to data from the Centers for Disease Control and Prevention.
¡°The point is there¡¯s still disease going on,¡± said Hayden, who has studied antiviral medicines for flu and other respiratory illnesses for about 50 years. ¡°There certainly is a place for an agent like this to be used for post-exposure prophylaxis.¡±
Separate research has shown Shionogi¡¯s ensitrelvir speeds clearance of the virus in infected patients, though it appeared somewhat less potent than Paxlovid in a 2025 study.
Ensitrelvir is already approved in Japan for treating mild-to-moderate COVID-19 and is under U.S. review for post-exposure use, with a decision expected next month.
With your current subscription plan you can comment on stories. However, before writing your first comment, please create a display name in the Profile section of your subscriber account page.