New Drug Ensitrelvir Shows Promise in Preventing COVID-19 Infection After Exposure

A recent study published in the New England Journal of Medicine reveals that the antiviral drug Ensitrelvir can significantly reduce the risk of COVID-19 infection when administered to individuals exposed to confirmed cases. Unlike vaccines, which do not fully prevent infection, Ensitrelvir offers a new preventive approach by targeting the viral enzyme responsible for processing proteins essential to the virus's replication.

Ensitrelvir, approved in Japan for treating COVID-19 symptoms, was tested in a clinical trial conducted between 2023 and 2024 across the United States, Japan, South Africa, Vietnam, and Argentina. The trial involved about 1,000 participants who had close contact with infected family members but were initially asymptomatic and PCR-negative. Participants who had recent COVID-19 infection or vaccination within six months were excluded.

The study found that only 2.9% of those treated with Ensitrelvir developed COVID-19 compared to 9% in the placebo group, indicating a reduction of over two-thirds in infection risk. Additionally, the drug delayed symptom onset in those who still became infected and halved the incidence of illness within a month after treatment compared to placebo. No significant side effects were reported.

Ensitrelvir works by inhibiting a viral enzyme that cleaves a large polyprotein into 16 functional proteins necessary for viral replication, thereby disrupting the virus's ability to multiply. The drug is manufactured by the Japanese pharmaceutical company Shionogi. In June 2026, the U.S. Food and Drug Administration approved Ensitrelvir for COVID-19 prevention, although it has not yet been authorized for use in Israel.

This development offers a new tool for protecting high-risk populations who are unvaccinated or insufficiently protected by vaccines, especially in household settings where exposure risk is high. The study supports the potential of Ensitrelvir as the first medication capable of preventing COVID-19 infection post-exposure.