Ensitrelvir for the Treatment of Hospitalized Adults With COVID-19: An International Phase 3 Randomized Placebo-controlled Trial

BACKGROUND: Antivirals remain an important treatment strategy for persons who experience severe and life-threatening COVID-19. Ensitrelvir is an oral 3CL protease inhibitor with potent antiviral activity. METHODS: We conducted an international randomized, placebo-controlled trial of ensitrelvir with standard of care (SOC) among adults hospitalized for COVID-19. The primary outcome was clinical recovery assessed by the days to recovery scale through day 60 (DRS-60), analyzed using a Van Elteren test. RESULTS: From 2023 to 2025, 589 participants received blinded study treatment (293 ensitrelvir and 296 placebo). Median age was 69 years, 49% were female, 68% were White, and SOC commonly included corticosteroids (61% and 54%) and remdesivir (62% and 60%) in ensitrelvir and placebo groups, respectively. Median DRS-60 category was 6 (IQR: 3-15) in the ensitrelvir and 5.5 (IQR: 3-12) in the placebo group (P = .19), and the OR was 0.82 (95% CI: 0.62-1.09) for a better DRS-60 category with ensitrelvir. Ensitrelvir participants had lower detectable viral antigen in plasma at Day 5 (13.4% vs 25.1%; P < .001). There was no difference in secondary clinical outcomes or prespecified safety outcomes, though the mortality rate was 6.1% vs 4.4% and the frequency of hemorrhagic events was 3.4% vs 0.3% among ensitrelvir and placebo groups, respectively. CONCLUSIONS: Ensitrelvir treatment did not improve clinical recovery in addition to SOC for adults hospitalized for COVID-19. The lower illness severity in the Omicron era compared with earlier periods in the COVID-19 pandemic and high use of remdesivir and corticosteroids, may have contributed to the lack of clinical benefit.