Shionogi & Co's (TYO:4507) oral antiviral ensitrelvir significantly reduced the risk of COVID-19 infection among household contacts of infected patients in a phase 3 post-exposure prophylaxis trial, according to study results published Wednesday.

The randomized, double-blind, placebo-controlled study enrolled more than 2,000 participants across Japan, assigning them to receive either ensitrelvir or placebo within 72 hours of symptom onset in an index case. By day 10, COVID-19 occurred in 2.9% of those receiving ensitrelvir versus 9.0% in the placebo group, corresponding to a risk reduction of about 67%.

The safety profile was broadly comparable between the two groups, with similar rates of adverse events and no COVID-19-related hospitalisations or deaths reported during the trial.

Ensitrelvir is already approved in Japan for the treatment of mild-to-moderate COVID-19, and the company said the findings support its potential use in preventing infection following household exposure.