Safety and Immunogenicity of a Messenger RNA–Based Cytomegalovirus Vaccine in Healthy Adults: Results From a Phase 1 Randomized Clinical Trial

Background This phase 1 trial evaluated the safety, reactogenicity, and immunogenicity of mRNA-1647, a messenger RNA (mRNA)–based cytomegalovirus (CMV) vaccine, in CMV-seronegative and -seropositive adults. Methods Participants were randomly assigned to receive 30, 90, 180, or 300 µg of mRNA-1647 or placebo on a 0-, 2-, and 6-month schedule and followed for 12 months after the last dose. Results A total of 154 (80 CMV-seronegative and 74 CMV-seropositive) participants were enrolled; 118 participants were randomized to mRNA-1647 and 36 to placebo. Mean (standard deviation) age was 32.5 (8.6) and 35.1 (8.9) years in the placebo and mRNA-1647 groups, respectively, in phase B (63% and 64% female) and 42.5 (6.2) and 33.3 (8.7) years, respectively, in phase C (2% and 16% female). No deaths, related serious adverse events, or adverse events of special interest were reported. Most adverse reactions were grade ≤2 severity. Increased neutralizing antibody, binding antibody, and antigen-specific cell-mediated responses were observed across mRNA-1647 treatment groups, regardless of CMV serostatus. Conclusions This phase 1, first-in-human trial demonstrated that mRNA-1647 has an acceptable safety profile in adults and elicits humoral and cellular immune responses. Clinical Trials Registration. NCT03382405.