A Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of mRNA-1283 in Participants ≥6 Months to <12 Years of Age

mRNA-1283 will be administered at 2 different dose levels (Dose Level 1 and Dose Level 2), with mRNA-1273 serving as the active comparator. Dose Level 1 will be the lower dose, and Dose Level 2 will be the higher dose.

The study will consist of 2 cohorts. In Cohort 1, mRNA-1283 will be administered as a single dose to participants aged ≥2 to <12 years, regardless of prior coronavirus disease 2019 (COVID-19) vaccination status, with mRNA-1273 included as a comparator. In Cohort 2, mRNA-1283 will be administered as a 2-dose series to participants aged ≥6 months to <2 years who are previously unvaccinated for COVID-19, with mRNA-1273 included as a comparator.

Key Inclusion Criteria:

• Participant is male or female, between ≥2 years to <12 years of age for Cohort 1 or ≥6 months to <2 years of age for Cohort 2 (calculated from date of birth) at the time of signing the informed consent/assent.
• Participant is in good general health as determined by medical evaluation including medical history and physical examination. Participants with chronic but stable conditions (eg, gastroesophageal reflux disease, asthma) may be eligible to enroll, provided the condition is well-controlled and not expected to interfere with study participation or outcomes.
• For participants in Cohort 2: the participant was born at ≥37 weeks, with a minimum birth weight of 2.5 kilograms, without fetal growth restriction.
• Female participants of nonchildbearing potential may be enrolled in the study. Nonchildbearing potential is defined as premenarche. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a participant with an early undetected pregnancy. If childbearing potential is unclear (for example, amenorrhea in athletes) and a menstrual cycle cannot be confirmed before first dose of study intervention, additional evaluation should be considered.
• Female participants who have reached menarche may be enrolled in the study if all of the following criteria are fulfilled:
• Has a negative highly sensitive pregnancy test (urine or serum) as required by local regulations within 24 hours before the study intervention.
• Is not currently chest/breastfeeding.
• Has been using a highly effective contraceptive method or has abstained from all activities that could lead to pregnancy for at least 28 days prior to the study injection (Day 1). The investigator should evaluate the potential for contraceptive method failure (for example, noncompliance, recently initiated) in relationship to the study injection.
• Has agreed to continue adequate contraception through the end of study. Contraceptive use by participants should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.

Key Exclusion Criteria:

• History of SARS-CoV-2 infection within 3 months prior to enrollment.
• Acutely ill or febrile (temperature ≥38.0℃elsius [100.4°Fahrenheit]) within 72 hours prior to or at the screening visit or Day 1. Participants who are acutely ill or febrile may be scheduled for a separate dosing visit within the protocol-specified window for screening and will retain their initially assigned participant number. Afebrile participants with minor illnesses can be enrolled at the discretion of the Investigator.
• Any medical, psychiatric, or occupational condition, including reported history of drug, or alcohol abuse in the parent(s)/ legally authorized representative(s), that, in the opinion of the investigator, might pose additional risk due to child's participation in the study, could interfere with safety assessments, or could interfere with the interpretation of study results.
• Receipt of the 2026-2027 formulation of a COVID-19 vaccine at any time prior to enrollment.
• Receipt of any COVID-19 vaccine within 6 months prior to enrollment.
• Receipt of any nonstudy vaccination within 14 days (28 days for live vaccines) prior to study intervention or plans to receive a nonstudy vaccine through 14 days after administration of study intervention.
• Received systemic immunosuppressive treatment, including long-acting biological therapies that affect immune responses (for example, infliximab), within 180 days prior to Day 1 (baseline) or plans to do so during the study.
• Received systemic immunoglobulins or blood products within 90 days prior to the screening visit or plans to receive them during the study. Exception: Nirsevimab and clesrovimab are allowed.
• For infants who receive breast milk, maternal receipt of systemic immunosuppressive therapy during lactation. This includes ≥10 milligrams/day of prednisone or equivalent for 14 days or more, or other immunomodulating agents (for example, biologics, calcineurin inhibitors, disease-modifying agents) that may be transferred via breastmilk and impact infant immune development or vaccine response.
• Has participated in an interventional clinical study within 28 days prior to the screening visit based on the medical history interview or plans to do so while participating in this study.
• Has been placed under the control or protection of an agency, organization, institution, or entity by the courts, the government, or a government body, acting in accordance with powers conferred on them by laws, and regulation. Does not include a child who is adopted or has an appointed legal guardian.
• Receipt of any nonstudy vaccination within 14 days (28 days for live vaccines) prior to study intervention or plans to receive a nonstudy vaccine through 14 days after administration of study intervention.
• History of any myocarditis, pericarditis, or myopericarditis

Note: Other inclusion and exclusion criteria may apply.