A Study to Investigate the Safety and Efficacy of mRNA-1403 in Participants ≥18 Years of Age for the Prevention of Acute Gastroenteritis

Inclusion Criteria:

1. Participants who understand and are willing and physically able to comply withprotocol mandated follow-up, including all procedures as assessed by theInvestigator, and who are primarily responsible for their self-care and activitiesof daily living.

2. Participants may have chronic medical diagnoses but should be medically stable asassessed by the following criteria:

• Absence of medical events associated with underlying diagnoses and qualifyingas SAEs within 1 month prior to study intervention on Day 1.

• Absence of known, current, and life-limiting diagnoses, which could continuefor the duration of the study and which, in the opinion of the Investigator,would make completion of the protocol unlikely.

3. Capable of giving signed informed consent, which includes compliance with therequirements and restrictions listed in the informed consent form (ICF) and in theprotocol.

4. Participants assigned female at birth are eligible to participate if they are notpregnant or breastfeeding/chestfeeding/bodyfeeding and meet protocol definedcriteria.

Exclusion Criteria:

1. Participant is acutely ill or febrile (temperature ≥38.0 degrees Celsius (°C) [100.4degrees Fahrenheit [°F]) 72 hours prior to or at the Screening Visit or Day 1.

2. History of AGE within 14 days prior to Day 1 or close contact with an individualwith AGE symptoms (in the home, socially, or occupationally) within 14 days prior toDay 1.

3. 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.

4. Current diagnosis of chronic gastrointestinal (GI) disease that is associated withongoing symptoms of regular vomiting and/or diarrhea. Participants with stable,well-controlled chronic GI disease without regular vomiting and/or diarrhea can beconsidered for enrollment.

5. Has a medical, psychiatric, or occupational condition that may pose additional riskas a result of participation or that could interfere with safety assessments orinterpretation of results according to the investigator's judgment.

6. Has undergone surgical procedures within 7 days prior to Day 1 or is scheduled toundergo a surgical procedure within 28 days after study intervention.

7. Reported history of congenital or acquired immunodeficiency,immunocompromising/immunosuppressive condition, asplenia, or recurrent severeinfections per protocol.

8. Reported history of anaphylaxis, urticaria, or other significant AR requiringmedical intervention after receipt of a vaccine or intervention that includes one ormore of the same components contained in the study vaccine.

9. History of myocarditis, pericarditis, or myopericarditis with onset within 180 daysprior to Day 1 whose values have not returned to Baseline clinical status.

10. History of Guillain-Barre syndrome.

11. Reported history of coagulopathy or bleeding disorder considered a contraindicationto IM injection or phlebotomy.

12. Dermatologic conditions that could affect local solicited AR assessments.

13. Diagnosis of malignancy within the previous 2 years (excluding nonmelanoma skincancer).

14. Has received systemic immunosuppressive therapies for >14 days in total within 6months prior to Day 1 (for corticosteroids ≥10 milligrams (mg)/day of prednisoneequivalent) or is anticipating the need for immunosuppressive treatment at any timeduring participation in the study.

15. Has received or plans to receive any licensed vaccine ≤28 days prior to or within 28days after study intervention (Day 1), except for influenza vaccines, which may begiven 14 days before or after receipt of a study intervention.

16. Receipt of systemic immunoglobulins or blood products within 3 months prior to Day 1or plans for receipt during the study.

17. Has donated ≥450 mL of blood products within 28 days prior to Day 1 Visit or plansto donate blood products during the study.

18. Participated in a clinical study with investigational treatment within 90 days priorto Day 1 (Baseline) based on the medical history interview or plans to do so whileparticipating in this study. Participants may continue in prior interventional studyfollow-up activities, provided that it does not involve further investigationaltreatment and/or procedures that may affect safety and/or other study endpoints, andstudy participation does not result in a breach of either study protocol.

19. Is working as, has worked as, or is an immediate family member or household memberof study personnel, Sponsor personnel, or study site staff directly involved withthe conduct of the study.