A Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1647 Cytomegalovirus (CMV) Vaccine

The main purpose of the study is to evaluate the efficacy of mRNA-1647 compared to placebo to prevent first clinically significant cytomegalovirus infection (CS-CMVi) in the period following cessation of CMV prophylactic treatment (such as letermovir) on Day 100 post- HCT through Month 9 post-HCT.

Inclusion Criteria:Receipt of an allogenic HCT.CMV-seropositive, defined as a positive test for anti-CMV IgG.High-risk for CMV: HCT from related, unrelated, or haploidentical donor with post-transplant cyclophosphamide for graft-versus-host-disease (GVHD) prophylaxis; OR HCT from related or unrelated donor with at least one mismatch at any of the following human leukocyte antigen (HLA) gene loci (HLA-A, B, C, and DRB1); OR HCT from related or unrelated donor with myeloablative conditioning.Female participants of nonchildbearing potential or of childbearing potential with negative urine or serum pregnancy test on the day of first vaccination.Exclusion Criteria:History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures.Treatment with alemtuzumab (Campath®), antithymocyte globulin (ATG), or any equivalent in-vivo T cell depleting agent within 12 months.HCT with ex-vivo T cell depletion.Low risk for CMV: HCT from related or unrelated donor with RIC and no other high-risk features.History of prior hematopoietic cell transplantation within 12 months Receipt of prior investigational CMV vaccines or participation in another CMV therapeutic trial that may interfere with study outcome measures as determined by the Investigator.Suspected or known allergic reaction to any component of any mRNA vaccine or its excipients.