Safety and Immunogenicity of the Cytomegalovirus Vaccine mRNA-1647 in Healthy Adults: Results from a Phase 2, Randomized, Controlled, Dose-Finding Trial with Long-Term Extension Follow-Up Through Month 48.

Background/Objectives: No licensed vaccine against cytomegalovirus (CMV) is currently available, despite the significant risk of mother-to-infant transmission leading to serious neurodevelopmental impairment and the substantial morbidity caused by CMV infection in immunocompromised persons. We report results from a phase 2 trial of the investigational CMV mRNA vaccine mRNA-1647 and a long-term extension study (NCT04232280; NCT04975893). Methods: This randomized, observer-blind, placebo-controlled phase 2 study, conducted at 9 US sites, enrolled participants in two parts. In the first part, healthy adults aged 18-40 years were stratified by baseline CMV status into CMV-seronegative and CMV-seropositive parallel cohorts and randomized 3:1 to receive mRNA-1647 (50, 100, or 150 μg) or placebo. In the second part, healthy female participants aged 18-40 years were randomized 3:1 to receive 100 μg mRNA-1647 or placebo. In both parts, vaccine or placebo was administered at Months 0, 2, and 6. Participants completing the Primary Trial through Month 18 were eligible to enroll in the extension study, wherein safety and immunogenicity were assessed every 6 months until all participants reached Month 48 (interim analysis) and a subset had Month 54 immunogenicity samples available. Primary objectives were to assess safety and neutralizing antibody responses. Results: Solicited adverse reactions were mostly grade 1 or 2 in severity, and no notable dose-related safety trends were identified. Neutralizing antibody and antigen-specific binding IgG responses were induced in CMV-seronegative participants and boosted in CMV-seropositive participants, with durability of responses through Month 48 and up to Month 54. Conclusions: The investigational vaccine mRNA-1647 was generally well tolerated and induced durable humoral immune responses across baseline CMV serostatus, with persistence supported through Month 48 and by available Month 54 data.