Key Inclusion Criteria:

- mRNA-1273.167 and mRNA-1273.712: Previously vaccinated adults aged ≥18 years

- mRNA-1273.251: Participants ≥65 years or ≥12 to <65 years of age with at least 1
risk factor for severe outcomes from COVID-19.

- Able to comply with study procedures based on the assessment of the Investigator.

- Female participants of childbearing potential may be enrolled in the study if the
participant fulfills all the following criteria:

- Has a negative pregnancy test at the Screening Visit and on the day of
vaccination prior to vaccine dose being administered on Day 1.

- Has practiced adequate contraception or has abstained from all activities that
could result in pregnancy for at least 28 days prior to the first dose.

- Has agreed to continue adequate contraception through 28 days following vaccine
administration.

Key Exclusion Criteria:

- History of SARS-CoV-2 infection within 3 months prior to enrollment for
mRNA-1273.167 and mRNA-1273.712 or within 6 months prior to enrollment for
mRNA-1273.251.

- Is acutely ill or febrile (temperature 38.0°Celsius/100.4°Fahrenheit) less than 72
hours prior to or at the Screening Visit or Day 1. Participants meeting this
criterion may be rescheduled within the Screening window.

- History of a diagnosis or condition that, in the judgment of the Investigator, is
clinically unstable, or may affect participant safety, assessment of study
endpoints, assessment of immune response, or adherence to study procedures.

- History of anaphylaxis or severe hypersensitivity reaction requiring medical
intervention after receipt of any mRNA vaccine or therapeutic or any components of
an mRNA vaccine or therapeutic.

- Receipt of COVID-19 vaccine within 3 months prior to enrollment for mRNA-1273.167
and mRNA-1273.712 or within 6 months prior to enrollment for mRNA-1273.251.

- Receipt of any licensed non-COVID-19 vaccine within 28 days before or planned
receipt within 28 days after the study intervention, except an influenza vaccine,
which may be given 14 days before or after receipt of the study intervention.

- Receipt of systemic immunosuppressants or immune-modifying drugs for >14 days total
(within 6 months prior to Screening for mRNA-1273.167 and mRNA-1273.712 or within
180 days prior to Screening for mRNA-1273.251) or is anticipating the need for
immunosuppressive treatment at any time during participation in the study.

- Receipt of systemic immunoglobulins or blood products (within 3 months prior to
Screening for mRNA-1273.167 and mRNA-1273.712 or within 90 days prior to the
Screening for mRNA-1273.251) or plans for receipt during the study.

- Has donated ≥450 milliliters of blood products within 28 days prior to the Screening
Visit or plans to donate blood products during the study.

Other protocol defined inclusion/exclusion criteria may apply.