A Study to Evaluate the Safety and Immunogenicity of the mRNA COVID-19 Vaccines in Healthy Children Between 6 Months to Less Than 6 Years of Age

Inclusion Criteria:

• The participant is 6 months to <6 years for Parts 1, 2, and 3; 2 years to <5 yearsfor Part 4A; and 6 months to <2 years for Part 4B at the time of consent (ScreeningVisit). Note: for Part 1, participant must be at least 6 months old and must nothave reached 6 years of age at the time of administration of first dose. For Part 4B, the participant must be at least 6 months old and must not have reached 2 yearsof age at the time of administration of first dose.

• If the participant has a chronic disease (that is, asthma, diabetes mellitus, cysticfibrosis, human immunodeficiency virus [HIV] infection), the disease should bestable, per investigator assessment, so that the participant can be consideredeligible for inclusion. Stable diseases are those which have had no change in theirstatus or in the medications required to control them in the 6 months prior toScreening Visit. Note: a change in medication for dose optimization (that is,insulin dose changes, adjustments for age-related weight gain), change within classof medication, or reduction in dose are not considered signs of instability.

• In the investigator's opinion, the parent(s)/legally authorized representative(s) (LAR[s]) understand and are willing and physically able to comply withprotocol-mandated follow-up, including all procedures and provide written informedconsent.

• The participant is 2 years or older and has a body mass index (BMI) at or above thesecond percentile according to World Health Organization (WHO) Child GrowthStandards at the Screening Visit.

OR

• The participant is less than 2 years of age and the participant's height and weightare both at or above the second percentile according to WHO Child Growth Standardsat the Screening Visit.

Special inclusion criteria for participants aged 6 months to < 12 months:

• The participant was born at full-term (≥ 37 weeks gestation) with a minimum birthweight of 2.5 kilograms (kg).

Inclusion criteria for Part 2:

• The participant must have received 2 doses of mRNA-1273, approximately 28 to 35 daysapart, as 25-μg primary series, and second dose was given at least 4 months prior toenrollment.

Inclusion criteria for Part 3 only:

• The participant must have received an age-appropriate immunization series of an authorized/approved COVID-19 vaccine, with the last dose given at least 4 months prior to enrolment (Previous vaccines NOT allowed are: XBB.1.5-containing formulation).

Exclusion Criteria:

• Has a known history of SARS-CoV-2 infection (that is, reported AE of COVID-19 orasymptomatic SARS-CoV-2 infection during Study mRNA-1273-P204 at the time ofrollover into mRNA-1273-P306 or during Part 1 at the time of rollover into Part 3)in the 90 days prior to dosing in this study.

• Is acutely ill or febrile 24 hours prior to or at the Screening Visit. Fever isdefined as a body temperature ≥ 38.0°Celsius (C)/≥ 100.4°Fahrenheit (F).Participants who meet this criterion may have visits rescheduled within the relevantstudy visit windows. Afebrile participants with minor illnesses can be enrolled atthe discretion of the investigator.

• For Parts 1 and 4, participant has previously been administered an investigationalor approved CoV (that is, SARS-CoV-2, SARS-CoV, Middle East respiratory syndromecoronavirus [MERS-CoV]) vaccine. For Part 2, participant who received anyapproved/investigational CoV vaccine are ineligible to participate except for thosewho received mRNA-1273 (prototype) vaccine.

• Has undergone treatment with investigational or approved agents for prophylaxisagainst COVID-19 (including receipt of SARS-CoV-2 monoclonal antibodies forprophylaxis or treatment) within 90 days prior to enrollment.

• Has a known hypersensitivity to a component of the vaccine or its excipients.Hypersensitivity includes, but is not limited to, anaphylaxis or immediate allergicreaction of any severity to a previous dose of an mRNA COVID-19 vaccine or any ofits components (including polyethylene glycol [PEG] or immediate allergic reactionof any severity to polysorbate).

• Has a medical or psychiatric condition that, according to the investigator'sjudgment, may pose additional risk as a result of participation, interfere withsafety assessments, or interfere with interpretation of results.

• Has a history of diagnosis or condition that, in the judgment of the investigator,may affect study endpoint assessment or compromise participant safety, specificallythe following:

• Congenital or acquired immunodeficiency, other than well-controlled HIVinfection.

• Chronic hepatitis or suspected active hepatitis

• A bleeding disorder that is considered a contraindication to IM injection orphlebotomy

• Dermatologic conditions that could affect local solicited AR assessments

• Any prior diagnosis of malignancy (excluding nonmelanoma skin cancer)

• Has received the following:

• Any routine vaccination with inactivated or live vaccine(s) within 14 daysprior to study vaccination or plans to receive such a vaccine through 14 daysfollowing study vaccination.

Note: This excludes influenza vaccine that may be given anytime, but ideally at least 7 days before study dose. If a participant receives an influenza vaccine, this should be captured within the concomitant medication electronic case report form (eCRF).

• Systemic immunosuppressants or immune-modifying drugs for >14 days in total within 6months prior to the day of enrollment (for corticosteroids, ≥1 milligrams (mg)/kg/day or ≥ 10 mg/day prednisone equivalent, if participant weighs >10 kg).Participants may have visits rescheduled for enrollment if they no longer meet thiscriterion within the Screening Visit window. Inhaled, nasal, and topical steroids,and palivizumab are allowed.

• Intravenous (IV) or subcutaneous (SC) blood products (red cells, platelets,immunoglobulins) within 3 months prior to enrollment.

Note: Other inclusion and exclusion criteria may apply.