A Study to Evaluate Safety and Effectiveness of mRNA-1273 COVID-19 Vaccine in Healthy Children Between 6 Months of Age and Less Than 12 Years of Age

The primary goal for this study is to evaluate up to 3 dose levels of mRNA-1273 vaccine given to healthy children as intramuscular (IM) injection in 2 doses (in Parts 1 and 2) and 3 doses (in Part 3), and a third dose or an optional booster dose (BD) (in Parts 1 and 2).

This is a Phase 2/3, 3-part, open-label, dose-escalation, age de-escalation, randomized, observer-blind, placebo-controlled, expansion study intended to infer the effectiveness of mRNA-1273 in children aged 6 months to less than 12 years.

Please access http://www.kidcovestudy.com for additional information, such as Study Overview, Participation, Site Locations along with contact numbers for each location for the study.

Eligibility Criteria:
Key Inclusion Criteria:

* For participants with chronic diseases (such as, asthma, diabetes mellitus, cystic fibrosis, human immunodeficiency virus \[HIV\] infection), the disease should be stable, per investigator assessment.
* Investigator assessment that the parent(s)/legally acceptable representatives understand and are willing and physically able to comply with protocol mandated follow-up, including all procedures, written informed consent is provided, and participants provide assent.
* For children 2 years of age or older has a body mass index at or above the third percentile according to World Health Organization (WHO) Child Growth Standards at the Screening Visit.
* For children 6 months to \<12 months of age: born at full-term with a minimum birth weight of 2.5 kilograms (kg).
* For female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection, agreement to continue adequate contraception or abstinence through 3 months following the second injection (Day 29) and the third dose in Part 3 (Day 149/booster dose Day 1), and not currently breastfeeding.

Key Exclusion Criteria:

* Known history of SARS-CoV-2 infection within 2 weeks prior to administration of vaccine or known close contact with anyone with laboratory-confirmed SARS-CoV-2 infection or COVID-19 within 2 weeks prior to administration of vaccine.
* Prior administration of an investigational or approved CoV (such as, SARS-CoV-2, SARS CoV, Middle East Respiratory Syndrome CoV) vaccine.
* Treatment with investigational or approved agents for prophylaxis against COVID 19 (such as, receipt of SARS-CoV-2 monoclonal antibodies) within 6 months prior to enrollment.
* Known hypersensitivity to a component of the vaccine or its excipients.
* A medical or psychiatric 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.
* History of a diagnosis or condition that, in the judgment of the investigator, may affect study endpoint assessment or compromise participant safety.
* Received any non-study vaccine within 14 days before or after any dose of vaccine (except for seasonal influenza vaccine, which is not permitted within 14 days before or after any dose of vaccine)
* Received intravenous or subcutaneous blood products (red blood cells, platelets, immunoglobulins) within 3 months prior to Day 1
* Participated in an interventional clinical study within 28 days prior to Day 0 or plans to donate blood products while participating in this study.