Dose-Finding Trial to Evaluate the Safety and Immunogenicity of Cytomegalovirus (CMV) Vaccine mRNA-1647 in Healthy Adults

This clinical study will assess the safety and immunogenicity of 3 dose levels of mRNA-1647 cytomegalovirus vaccine in CMV-seronegative and CMV-seropositive healthy adults 18-40 years of age.

mRNA-1647-P202 is a 2-part study. Part 1 of the study evaluates the safety and immunogenicity of 3 dose levels of mRNA-1647 vaccine or placebo, administered on a 0, 2, 6-month schedule in healthy CMV-seronegative and CMV-seropositive males and females, 18 to 40 years of age. A planned interim analysis of safety and immunogenicity through Month 3 (1 month after the second dose) of Part 1 of the study informed the selection of the middle dose level for further development. Part 2 of the study is designed to further evaluate the safety and immunogenicity of the middle dose level of mRNA-1647 vaccine or placebo on a 0, 2, 6-month schedule in approximately 200 healthy participants 18 to 40 years of age, comprised of CMV-seronegative and CMV-seropositive female population, which includes the target population for the pivotal Phase 3 efficacy trial.

Eligibility Criteria:
Inclusion Criteria:

* Male or female 18-40 years of age (Part 1); Female 18-40 years of age (Part 2)
* Understands and agrees to comply with the trial procedures and provides written informed consent
* According to the assessment of the Investigator, is in good general health and is capable of complying with trial procedures
* Body mass index (BMI) 18-35 kilograms/meter (kg/m\^2)
* Female participants must either be of non-childbearing potential or use acceptable methods of contraception from at least 28 days prior to the first vaccination and through 3 months following last vaccination and is not breastfeeding.
* Male participants must agree to practice adequate contraception from the time of the first vaccination and through 3 months after the last vaccination.

Exclusion Criteria:

* Acutely ill or febrile on the day of the first vaccination
* Prior receipt of any CMV vaccine
* Abnormal screening safety laboratory test results
* Diagnosis or condition that, in the judgment of Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to trial procedures
* Has received or plans to receive a vaccine ≤28 days prior to the first vaccination or plans to receive a non-study vaccine within 28 days prior to or after any study vaccination, except for any licensed influenza vaccine which can be administered \>14 days before or after any study vaccination. COVID-19 vaccines (regardless of manufacturer) may be administered \>7 days but preferably \>14 days before or after any study vaccination, with the intention of prioritizing COVID-19 vaccination over all other considerations.
* Prior receipt of chronic systemic immunosuppressants or immune-modifying drugs
* Receipt of intravenous immunoglobulins or plasma products within 3 months prior to the day of the first study vaccination
* Previous receipt of medications in lipid nanoparticle (LNP) formulation (Part 1 participants only)
* Has donated ≥450 milliliters (mL) of blood products within 28 days of the Screening visit
* Participated in an interventional clinical trial within 28 days prior to the day of enrollment
* Is an immediate family member or household member of trial personnel