The trial was terminated because data from the dose finding part of the trial did not support further evaluation of effectiveness of mRNA-1273. There were no safety concerns.

The study evaluated the safety, tolerability, reactogenicity, and effectiveness of mRNA-1273.214 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine in infants aged 12 weeks to < 6 months.

The purpose of this pediatric study was to confirm safety and effectiveness of mRNA-1273.214 in infants between 12 weeks to < 6 months of age by comparing the immune response of infants in this study to adults (>18 years of age) enrolled in the mRNA-1273-P301 study [NCT04470427]).

The study was planned to be conducted in 2 parts. Part 1 was open-label and evaluated 2 dose levels. The dose level selected from Part 1 was planned to be further evaluated in Part 2. However, data from the dose finding part of the trial (Part 1) did not support further evaluation of effectiveness of mRNA-1273 and accordingly, Part 2 of the study was not conducted and the trial was terminated. There were no safety concerns.

Key Inclusion Criteria:

• Participant is male or female, between 2 and <6 months of age at the time of consent (Screening Visit), who is in good general health, in the opinion of the investigator, based on review of medical history and screening physical examination.

  1. Participant must be at least 12 weeks completed age and must not have completed 6 months at the time of administration of first dose.
  2. If the participant has a chronic, stable disease, they may be eligible to enroll in Part 2, but ineligible for Part 1. The chronic condition (for example, gastroesophageal reflux disease) should be stable, per investigator assessment, so that the participant can be considered eligible for inclusion in Part 2.

• Participant was born at ≥37 weeks gestation (Part 1) or ≥34 weeks gestation (Part 2), with a minimum birth weight of 2.5 kilograms (kg), without fetal growth restriction, and the participant's height and weight are both at or above the second percentile for age according to the Centers for Disease Control and Prevention/World Health Organization Child Growth Standard at the Screening Visit.

• In the investigator's opinion, the parent(s)/legally authorized representative(s) understand and are willing and physically able to comply with protocol-mandated follow-up, including all procedures, and provide written informed consent.

Key Exclusion Criteria:

• Participant has a known history of SARS-CoV-2 infection within 2 weeks prior to administration of study drug or has a known close contact in the past 2 weeks to someone diagnosed with SARS-CoV-2 infection or coronavirus disease 2019 (COVID-19). Participants may be rescreened after 14 days provided that they remain asymptomatic.

• Participant is acutely ill or febrile 72 hours prior to or at the Screening Visit. Fever is defined as a body temperature ≥38.0°Celcius/≥100.4°Farenheit. Participants who meet this criterion may have visits rescheduled within the relevant study visit windows. Afebrile participants with minor illnesses can be enrolled at the discretion of the investigator.

• Participant has previously been administered an investigational or approved CoV (for example, SARS-CoV-2, SARS-CoV, Middle East respiratory syndrome [MERS]-CoV) vaccine.

• Participant has undergone treatment with investigational or approved agents for prophylaxis against COVID-19 (for example, receipt of SARS-CoV-2 monoclonal antibodies) within 90 days prior to enrollment.

• Participant has a known hypersensitivity to a component of the vaccine or its excipients. Hypersensitivity includes, but is not limited to, anaphylaxis or immediate allergic reaction of any severity to any of the components of messenger ribonucleic acid (mRNA) COVID-19 vaccines (including polyethylene glycol or immediate allergic reaction of any severity to polysorbate).

• Participant has a medical, psychiatric, or occupational condition, that, according to the investigator's judgment, may pose additional risk as a result of participation, interfere with safety assessments, or interfere with interpretation of results.

• Participant has a history of diagnosis or condition that, in the judgment of the investigator, may affect study endpoint assessment or compromise participant safety.

• Participant has received the following:

  1. Any routine vaccination with inactivated or live vaccine(s) within 14 days prior to first or second vaccination or plans to receive such a vaccine within 14 days of any study vaccination.
  2. Systemic immunosuppressants or immune-modifying drugs (including maternal use during pregnancy or lactation) for >14 days in total within 6 months prior to the day of enrollment (for corticosteroids, ≥ 1 milligrams (mg)/kg/day or, if participant weighs >10 kg: ≥10 mg/day prednisone equivalent). Participants may have visits rescheduled for enrollment if they no longer meet this criterion within the Screening Visit window. Inhaled, nasal, and topical steroids are allowed.
  3. Intravenous or subcutaneous blood products (red blood cells, platelets, immunoglobulins) within 3 months prior to enrollment.

• Participant has participated in an interventional clinical study within 28 days prior to the Screening Visit or plans to do so while participating in this study, or maternal participation in an interventional clinical study during pregnancy.

Note: Other inclusion and exclusion criteria may apply.

Part 1: open-label Part 2: observer-blinded, randomized, placebo-controlled