Ensitrelvir is approved in Japan.
A pill called ensitrelvir prevented COVID-19 in people who were exposed to infected individuals, according to a new study.
Just 2.9 percent of people in a trial who received the pill contracted COVID-19, compared to 9 percent in a placebo group, researchers with the University of Virginia School of Medicine and other institutions said in a paper published by the New England Journal of Medicine that went over the study results.
The trial was randomized and double-blind in addition to being placebo-controlled.
It tested ensitrelvir, which is made by a Japanese company, in individuals who lived with people who contracted COVID-19. Household contacts of COVID-19 patients received either ensitrelvir for five days, or a placebo shortly after the contacts started showing COVID-19 symptoms. The contacts tested negative for COVID-19 at the start of the trial.
Some 1,030 people received ensitrelvir, and 1,011 received a placebo.
Participants were counted as having COVID-19 if they tested positive for the illness and had at least one of 14 COVID-19 symptoms, such as fever and sore throat, that lasted at least 48 hours, or if they experienced a worsening of any preexisting symptoms.
Thirty pill recipients contracted COVID-19. Ninety-one placebo recipients contracted COVID-19.
“This is the first clinical trial of an oral antiviral drug to show significant protection against COVID-19,” Dr. Frederick Hayden, a University of Virginia School of Medicine emeritus professor who helped design the trial, said in a statement.
No participants had COVID-19-related hospitalizations or deaths.
The trial was conducted from June 2023 to September 2024.
Researchers said that adverse events, and serious adverse events, were similar between the two arms.
Ensitrelvir is approved in Japan to treat mild-to-moderate COVID-19 and as a post-exposure prophylaxis of COVID-19. It is known as Xocova in Japan.
Shionogi, the Japanese manufacturer that makes ensitrelvir, filed an application for clearance with the U.S. Food and Drug Administration in 2025. The company asked for clearance for the drug for the prevention of COVID-19 following exposure to an infected person.
The FDA said it planned to respond to the application by June 17, 2026.
