LC16m8 pre-exposure prophylaxis showed no mpox cases in a randomized controlled trial of high-risk adults

BACKGROUND: This randomized controlled trial provided LC16m8 pre-exposure prophylaxis to high-risk individuals to assess its efficacy for mpox prevention, safety, and immunogenicity. METHODS: This multicenter, randomized, open-label trial enrolled men and women aged ≥18 years at high risk of mpox. Participants were randomly assigned 1:1 to early- or late-vaccination groups. The primary endpoint was vaccine efficacy (VE) against mpox. Secondary endpoints included VE against severe mpox, symptoms, "take" incidence, adverse events (AEs), and immunogenicity in participants with human immunodeficiency virus (HIV). RESULTS: In total, 570 and 565 participants were assigned to early- and late-vaccination groups, respectively, with 530 and 476 vaccinated. The median age was 41 years; 99.7% were male, 89.7% were Japanese, and 34.4% had HIV. No mpox cases occurred, precluding VE calculations. The take rates were 89.5% (with HIV) and 93.9% (without HIV). AEs occurred in 97.2% and 98.2% of participants with and without HIV, respectively. No fatal AEs were observed. Serious adverse events (SAEs) were observed in 2/352 (0.6%) and 3/654 (0.5%) of participants with and without HIV, respectively, of which 1 SAE causally related to vaccination occurred in a participant without HIV. Seroconversion rates for LC16m8 and MPXV were 96.2% and 69.2%, respectively, in participants with HIV, and 92.0% and 52.0%, respectively, in individuals without HIV. CONCLUSIONS: LC16m8 efficacy in mpox remains inconclusive. However, in individuals with well-controlled HIV, it was immunogenic and raised no significant safety concerns, suggesting its suitability for targeted vaccination of at-risk groups. (Japan Registry of Clinical Trials number, jRCT1031230137).