A Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1647 Cytomegalovirus (CMV) Vaccine in Healthy Participants 16 to 40 Years of Age

The main purpose of this study is to evaluate the efficacy of mRNA 1647 vaccine in CMV-seronegative female participants and to evaluate the safety and reactogenicity of mRNA-1647 vaccine in all participants. The purpose of the Phase 3 extension substudy is to extend the observation period of the main study and to assess the longer-term immunogenicity, efficacy, and safety of the mRNA-1647 vaccine against primary CMV infection in healthy females who were CMV-seronegative at Baseline of the mRNA-1647-P301 main study (including participants who remain CMV-seronegative upon entry into the extension substudy and participants who seroconverted during the main study). The extension substudy will also evaluate the immune persistence and safety of mRNA-1647 in a subset of female participants who were CMV-seropositive at Baseline of the main study. No interventional vaccine will be administered in the extension substudy.