Oral antiviral drug proves successful in preventing symptomatic COVID-19 in household members

An international clinical trial has identified a medication that can help prevent symptomatic COVID-19 in people exposed within households. The results could be particularly important for families where someone is at high risk of serious complications from the illness.

The drug, ensitrelvir, is already approved in Japan to treat mild to moderate COVID cases. The SCORPIO-PEP trial, however, has demonstrated that it also has the potential to protect against illness. Uninfected people who began taking the antiviral within 72 hours of symptoms first appearing in a household member were significantly less likely to contract COVID-19 than people who were given an inactive placebo. 

Among the more than 2,000 trial participants in the United States and abroad, 9% of the participants taking the placebo developed confirmed cases of COVID-19. That's compared with only 2.9% of people taking ensitrelvir – a 67% reduction in risk of COVID-19 illness.

"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 and presented the trial results today at the Conference on Retroviruses and Opportunistic Infections (CROI) in San Francisco. 

Conducted between June 2023 and September 2024, the SCORPIO-PEP trial was double-blinded, meaning neither the researchers nor the uninfected household participants knew whether they were receiving ensitrelvir or placebo. Household members who took a 5-day course of ensitrelvir after exposure to a proven COVID illness were much less likely to get COVID but no more likely to suffer adverse events than those who took placebo. 

Among the trial participants, 37% had at least one risk factor for developing serious (and potentially life-threatening) complications. Their risk of developing COVID was significantly reduced, but there were no hospitalizations or deaths in either group.

In the summary of their findings presented today, the researchers describe ensitrelvir as effective and generally well tolerated for COVID-19 prevention. Before the drug could become available in the United States, however, the federal Food and Drug Administration would have to approve it for that purpose. Hayden and his collaborators say their results are an essential step in that process.

This is the first clinical trial of an oral antiviral drug to show significant protection against COVID-19. If approved by the Food and Drug Administration for this purpose, it would be an important addition to current preventive strategies." 

Frederick G. Hayden, MD, Division of Infectious Diseases and International Health, UVA School of Medicine