Shionogi's pharmaceutical research center in the city of Toyonaka, Osaka Prefecture. The firm's COVID-19 pill cut the risk of infection among people exposed to the disease at home, according to a recent study. | Bloomberg An experimental COVID-19 pill developed by Japan's Shionogi cut the risk of infection among people exposed to the disease at home, according to a study that could revive interest in antivirals as a preventive tool. In a late-stage trial of more than 2,000 household contacts, 2.9% of those given Shionogi’s once-daily pill ensitrelvir developed symptomatic COVID-19 within 10 days, compared with 9.0% on placebo — a roughly two-thirds reduction in risk. The study, published Wednesday in The New England Journal of Medicine, tested a five-day course started within 72 hours after a household member fell ill. Participants were initially virus-negative and most had some prior immunity through vaccination or earlier infection.…