BACKGROUND: Ensitrelvir, a severe acute respiratory syndrome coronavirus-2 main protease inhibitor, has demonstrated clinical and virologic efficacy in previous studies. METHODS: In this global phase 3 trial, nonhospitalized adults with mild-to-moderate coronavirus disease 2019 (COVID-19) and symptom onset within 5 days were randomized (1:1) to receive once-daily ensitrelvir (375 mg day 1, 125 mg days 2-5) or blinded matching placebo. The primary endpoint was the restricted mean time to sustained (≥2 days) resolution of 15 COVID-19 symptoms, recorded in participant daily diaries, through day 29 in participants starting treatment within 3 days after symptom onset. Virologic efficacy and safety were assessed. RESULTS: Of 2093 participants, 1888 started treatment within 3 days after symptom onset. Mean time to symptom resolution was 12.5 and 13.1 days with ensitrelvir and placebo, respectively (difference, -0.6 days
95% confidence interval, -1.38 to 0.19
P = .14). On day 4, ensitrelvir reduced least-squares mean RNA by 0.72 log10 copies/mL more than placebo (95% confidence interval, 0.55-0.90). Among those with positive viral cultures at enrollment, 274/287 (95.5%) ensitrelvir-treated versus 210/280 (75.0%) placebo-treated participants had negative cultures on day 4. RNA rebound was similar (<
1.5%) between groups. The proportion of participants with ≥1 adverse event was similar with ensitrelvir (61.5%) and placebo (60.6%). No treatment-related serious adverse events or deaths occurred. Three (0.3%) ensitrelvir-treated and 1 (0.1%) placebo-treated participants had COVID-19-related hospitalizations by day 29. CONCLUSIONS: Despite the evidence of antiviral activity with ensitrelvir, this trial did not demonstrate a significant difference in time to sustained symptom resolution.