A Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1647 Cytomegalovirus (CMV) Vaccine in Allogenic Hematopoietic Cell Transplantation (HCT) Participants.

Inclusion Criteria:

• Receipt of an allogeneic HCT.

• CMV-seropositive, defined as a documented positive test for anti-CMV IgG.

• High-risk for CMV: HCT from related, unrelated, or haploidentical donor withpost-transplant cyclophosphamide for graft-versus-host-disease (GVHD) prophylaxis;or HCT from related or unrelated donor with at least one mismatch at any of thefollowing human leukocyte antigen (HLA) gene loci (HLA-A, B, C, and DRB1); or HCTfrom related or unrelated donor with myeloablative conditioning.

• Persons of nonchildbearing potential or of childbearing potential with negativeurine or serum pregnancy test on the day of first study injection.

• Persons of childbearing potential who have practiced adequate contraception or haveabstained from all activities that could result in pregnancy for at least 28 daysprior to the first dose.

• Persons of childbearing potential who have agreed to continue adequate contraceptionor abstain from all activities that could result in pregnancy through 3 months afterlast study injection.

• Persons who are not currently breast/chestfeeding.

• Willingness to comply with study procedures and provide written informed consent.

Exclusion Criteria:

• History of a diagnosis or condition that, in the judgment of the Investigator, mayaffect participant safety, assessment of safety endpoints, assessment of immuneresponse, or adherence to study procedures.

• A documented positive human immunodeficiency virus (HIV) test.

• Treatment with alemtuzumab (Campath®), antithymocyte globulin (ATG), or anyequivalent 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 reduced intensityconditioning (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 CMVtherapeutic study that may interfere with study outcome measures as determined bythe Investigator.

• Suspected or known allergic reaction to any component of any mRNA vaccine or itsexcipients.