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

• mRNA-1403 (Biological): Intramuscular (IM) injection.
• Placebo (Biological): 0.9% sodium chloride solution by IM injection.

Primary outcomes

• Number of Participants with Solicited Local and Systemic Adverse Reactions (ARs) [Day 1 up to Day 7 (7 days after study intervention)]
• Number of Participants with Unsolicited Adverse Events (AEs) [Day 1 up to Day 28 (28 days after study intervention)]
• Number of Participants with Medically Attended Adverse Events (MAAEs) [Day 1 through Day 181]
• Number of Participants with Serious Adverse Events (SAEs), AEs of Special Interest (AESIs), and AEs Leading to Study Withdrawal [Day 1 up to Day 546]
• Vaccine Efficacy (VE) of mRNA-1403 to Prevent First Occurrence of Protocol-defined Moderate or Severe AGE Associated with Vaccine Matched Genotypes [Day 15 up to 9 months after last participant dosed]

Secondary outcomes

• VE of mRNA-1403 to Prevent First Occurrence of Protocol-defined Severe AGE Associated with Vaccine Matched Genotypes [Day 15 up to 9 months after last participant dosed]
• VE of mRNA-1403 to Prevent First Occurrence of Protocol-defined Severe Norovirus (NoV) AGE Associated with Any Genogroup I or II Norovirus (NoV) Genotype [Day 15 up to 9 months after last participant dosed]
• VE of mRNA-1403 to Prevent First Occurrence of Protocol-defined Moderate or Severe AGE Associated with Any Genogroup I or II NoV Genotype [Day 15 up to 9 months after last participant dosed]
• VE of mRNA-1403 to Prevent First Occurrence of Protocol-defined AGE of Any Severity Associated with Vaccine Matched NoV Genotypes [Day 15 up to 9 months after last participant dosed]
• VE of mRNA-1403 to Prevent Any Occurrence of Medically Attended AGE Associated with Vaccine Matched NoV Genotypes, as well as any Genogroup I or II NoV Genotype [Day 15 up to Day 546]
• VE of mRNA-1403 to Prevent Any Occurrence of AGE Hospitalizations Associated With Vaccine Matched NoV Genotypes, as well as any Genogroup I or II NoV Genotype [Day 15 up to Day 546]
• VE of mRNA-1403 to Prevent Any Occurrence of Protocol-defined Moderate or Severe AGE Associated with Vaccine Matched Genotypes [Day 15 up to Day 546]
• VE of mRNA-1403 to Prevent Any Occurrence of Protocol-defined AGE of Any Severity Associated with Any Genogroup I or II NoV Genotype [Day 15 up to Day 546]
• Geometric Mean Titer (GMT) of Histo-blood Group Antigen (HBGA)-blocking Antibodies Titers Against Vaccine Matched Genotypes [Day 1 and Day 29]
• Geometric Mean Fold Rise (GMFR) of HBGA-blocking Antibodies Titers Against Vaccine Matched Genotypes [Day 1 and Day 29]
• Percentage of Participants with Seroresponse Based on HBGA-blocking Antibody Titer [Day 29]
• GMT of Binding Antibody (bAb) Against Vaccine-matched NoV Genotypes [Day 1 and Day 29]
• GMFR of bAb Titers [Day 1 and Day 29]
• Percentage of Participants with Seroresponse Based on bAb Titer [Day 29]
• VE of mRNA-1403 to Prevent First Occurrence of Protocol-Defined Moderate or Severe AGE Associated with Vaccine-Matched NoV Genotypes Up to 12 Months [Day 15 up to 12 months postdose]
• VE of mRNA-1403 to Prevent First Occurrence of Protocol-Defined Moderate or Severe AGE Associated with Vaccine-Matched NoV Genotypes Up to 18 Months [Day 15 up to 18 months postdose]

Sex: All, ages: 18 Years to any, accepts healthy volunteers

Inclusion Criteria:

1. Participants who understand and are willing and physically able to comply with protocol mandated follow-up, including all procedures as assessed by the Investigator, and who are primarily responsible for their self-care and activities of daily living.
2. Participants may have chronic medical diagnoses but should be medically stable as assessed by the following criteria:

   * Absence of medical events associated with underlying diagnoses and qualifying as SAEs within 1 month prior to study intervention on Day 1.
   * Absence of known, current, and life-limiting diagnoses, which could continue for 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 the requirements and restrictions listed in the informed consent form (ICF) and in the protocol.
4. Participants assigned female at birth are eligible to participate if they are not pregnant or breastfeeding/chestfeeding/bodyfeeding and meet protocol defined criteria.

Exclusion Criteria:

1. Participant is acutely ill or febrile (temperature ≥38.0 degrees Celsius (°C) \[100.4 degrees 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 individual with AGE symptoms (in the home, socially, or occupationally) within 14 days prior to Day 1.
3. 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.
4. Current diagnosis of chronic gastrointestinal (GI) disease that is associated with ongoing symptoms of regular vomiting and/or diarrhea. Participants with stable, well-controlled chronic GI disease without regular vomiting and/or diarrhea can be considered for enrollment.
5. Has a medical, psychiatric, or occupational condition that may pose additional risk as a result of participation or that could interfere with safety assessments or interpretation of results according to the investigator's judgment.
6. Has undergone surgical procedures within 7 days prior to Day 1 or is scheduled to undergo a surgical procedure within 28 days after study intervention.
7. Reported history of congenital or acquired immunodeficiency, immunocompromising/immunosuppressive condition, asplenia, or recurrent severe infections per protocol.
8. Reported history of anaphylaxis, urticaria, or other significant AR requiring medical intervention after receipt of a vaccine or intervention that includes one or more of the same components contained in the study vaccine.
9. History of myocarditis, pericarditis, or myopericarditis with onset within 180 days prior to Day 1 whose values have not returned to Baseline clinical status.
10. History of Guillain-Barré syndrome.
11. Reported history of coagulopathy or bleeding disorder considered a contraindication to 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 skin cancer).
14. Has received systemic immunosuppressive therapies for >14 days in total within 6 months prior to Day 1 (for corticosteroids ≥10 milligrams (mg)/day of prednisone equivalent) or is anticipating the need for immunosuppressive treatment at any time during participation in the study.
15. Has received or plans to receive any licensed vaccine ≤28 days prior to or within 28 days after study intervention (Day 1), except for influenza vaccines, which may be given 14 days before or after receipt of a study intervention.
16. Receipt of systemic immunoglobulins or blood products within 3 months prior to Day 1 or plans for receipt during the study.
17. Has donated ≥450 mL of blood products within 28 days prior to Day 1 Visit or plans to donate blood products during the study.
18. Participated in a clinical study with investigational treatment within 90 days prior to Day 1 (Baseline) based on the medical history interview or plans to do so while participating in this study. Participants may continue in prior interventional study follow-up activities, provided that it does not involve further investigational treatment and/or procedures that may affect safety and/or other study endpoints, and study 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 member of study personnel, Sponsor personnel, or study site staff directly involved with the conduct of the study.

183 more sites not listed.