Ensitrelvir, an oral inhibitor of SARS-CoV-2 3C-like protease that is approved in some countries for treatment of COVID-19, was investigated as post-exposure prophylaxis in a phase 3, randomised, placebo-controlled trial in household contacts of individuals with COVID-19. Within 72 h of symptom onset in the index patient, 2041 contacts with a mean age of 42·4 years received 5 days’ treatment of either ensitrelvir (375 mg on day 1 and 125 mg on days 2–5; n=1030) or matched placebo (n=1011). The incidence of RT-PCR-confirmed COVID-19 in household contacts by day 10 was significantly lower with ensitrelvir than placebo: 2·9% versus 9·0% (risk ratio [RR] 0·33, 95% CI 0·22–0·49; p<0·001), suggesting its efficacy in preventing transmission.
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