As any doctor or scientist will tell you, vaccinations are our best line of defense against COVID-19. However, the vaccines are not foolproof, and certain people may be unable to take them. Medicines like Paxlovid exist to treat COVID in high-risk people by preventing the virus from multiplying if taken at the onset of an infection. But now scientists are touting the efficacy of a new drug that can stop COVID before it even starts if you've been exposed.
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What is Ensitrelvir?
Ensitrelvir is an oral drug currently approved in Japan (where it's known as Xocova) to treat mild and moderate COVID-19 cases, notes a press release.
According to the University of Colorado Anschutz Medical Campus, it is a "coronavirus protease inhibitor therapy" like Paxlovid. Both medicines specify that they must be taken within 72 hours of the onset of a COVID infection.
However, the SCORPIO-PEP trial, which was conducted between June 2023 and September 2024, found that Ensitrelvir can greatly reduce the risk of contracting COVID in uninfected people who take the drug within 72 hours of symptoms first appearing in a household member—meaning if you think you've been exposed to the virus, this medicine could stop it from developing at all.
"In addition to vaccination, post-exposure prophylaxis with timely use of an oral antiviral would be a valuable way to help prevent COVID-19 illness in people who have been exposed, especially people at high risk for severe disease," said University of Virginia School of Medicine emeritus professor Frederick G. Hayden, MD, who helped design the trial, in a statement.
For more context, the double-blinded SCORPIO-PEP trial included 2,041 household exposure participants in the U.S. and abroad, 37 percent of whom "had at least one risk factor for developing serious (and potentially life-threatening) complications," according to the release. Of the participants who took a placebo after household exposure, 9 percent developed a COVID infection. Only 2.9 percent of those who took a five-day course of Ensitrelvir became infected—a 67 percent reduction in illness.
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Could Ensitrelvir become available in the U.S.?
"This is the first clinical trial of an oral antiviral drug to show significant protection against COVID-19," said Hayden. "If approved by the Food and Drug Administration (FDA) for this purpose, it would be an important addition to current preventive strategies."
Ensitrelvir remains "an investigational drug" outside of Japan. However, the FDA granted the drug Fast Track designation in 2025 "for post-exposure prophylaxis of COVID-19 following contact with an individual who has COVID-19" and Fast Track designation in 2023 "for the treatment of COVID-19," according to the press release.
It remains to be seen when the drug could be approved and what, if any, additional documentation the FDA is seeking from the drug maker.
