Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of mRNA-1403 and mRNA-1405 to Prevent Norovirus Acute Gastroenteritis in Healthy Adults 18 to 80 Years of Age

Key Inclusion Criteria:

• Phase 1 Part: Adults 18 to 49 years of age at the time of consent or adults 60 to 80years of age at the time of consent.

• Phase 2 Part: Adults 18 to 80 years of age at the time of consent.

• Body mass index of 18 to 39 kilogram/meter^2 (inclusive) at the Screening Visit (relevant only for the base period of Phase 1 and Phase 2).

• Participants assigned female at birth are eligible to participate if they are notpregnant or breast/chest/body feeding.

• A person of childbearing potential is using a highly effective contraceptive method.

• Participant has provided written informed consent for participation in this study.

Key Exclusion Criteria:

• Is acutely ill or febrile (temperature ≥38.0°C [100.4°F]) 72 hours prior to or atthe Screening Visit or Day 1.

• Phase 2 Part booster extension period: acute disease or febrile 72 hours prior to oron Day 365.

• History of a diagnosis or condition that, in the judgment of the Investigator, isclinically unstable or may affect participant safety, assessment of safetyendpoints, assessment of immune response, or adherence to study procedures.

• Has a medical, psychiatric, or occupational condition, including reported history ofsubstance abuse, which may pose additional risk as a result of participation, orthat could interfere with safety assessments or interpretation of results accordingto the Investigator's judgment.

• Has a current or previous diagnosis of immunosuppressive condition, to include humanimmunodeficiency virus, or immune-mediated (autoimmune) disease that requiresimmunosuppressive therapies.

(drugs or biologics). The use of topical corticosteroids or other immunosuppressive agents (for example, topical calcineurin inhibitor) may be eligible for participation at the discretion of the Investigator.

• History of anaphylaxis, urticaria, or other significant AR requiring medicalintervention after receipt of a vaccine or intervention that includes one or more ofthe same components contained in the study vaccine.

• Phase 1 Part and Phase 2 Part base period: Any history of myocarditis orpericarditis.

• Phase 2 Part booster extension period: Any history of myocarditis, pericarditis, orGuillain-Barré Syndrome.

• Coagulopathy or bleeding disorder considered a contraindication to IM injection orphlebotomy.

• Dermatologic conditions that could affect local solicited AR assessments (forexample, tattoos, psoriasis patches affecting skin over the deltoid areas).

• Has received systemic immunosuppressive therapies (drugs or biologics) for >14 daysin total within 6 months prior to Screening or Day 365 (Phase 2 Part boosterextension period only) (for corticosteroids ≥10 mg/day of prednisone equivalent) oris anticipating the need for immunosuppressive treatment at any time duringparticipation in the study. Inhaled, nasal, intra-articular, and topical steroidsare allowed.

• Has received or plans to receive any licensed/authorized vaccine (to include severeacute respiratory syndrome-related coronavirus-2 vaccine) ≤28 days prior to orwithin 28 days after each study intervention, with the exception of influenzavaccines, which may be given 14 days before or after receipt of a studyintervention.

• Has received systemic immunoglobulins or blood products within 3 months prior toScreening or Day 365 (Phase 2 Part booster extension period only) or plans forreceipt during the study.

• Has donated protocol-specified volume of blood products within protocol-specifiedtimeframe.

• Participated or plans to participate in an interventional clinical study withinprotocol-specified timeframe.

• Phase 2 Part: Individuals who are participating/have participated in the Phase 1part of the study are excluded.

• Phase 1 Part: Any individual with laboratory abnormalities achieving thresholdsdefined in the protocol.