A Study to Evaluate the Safety, Reactogenicity, and Effectiveness of mRNA-1273 Vaccine in Adolescents 12 to <18 Years Old to Prevent COVID-19

Inclusion Criteria:

For Part 1A, Part 2 and Part 3:

- Participants 12 to <18 years of age at the time of consent (Screening Visit, Day 0)
who, in the opinion of the Investigator, are in good general health based on review of
medical history and screening physical examination.

- Investigator assessment that the participant, in the case of an emancipated minor, or
parent(s)/legally acceptable representative(s) (LAR[s]) understand and is willing and
physically able to comply with protocol-mandated follow up, including all procedures
and provides written informed consent/assent.

- Body mass index (BMI) at or above the third percentile according to World Health
Organization (WHO) Child Growth Standards at the Screening Visit (Day 0)

- Female participants of nonchildbearing potential may be enrolled in the study.
Nonchildbearing potential is defined as premenarche or surgically sterile (history of
bilateral tubal ligation, bilateral oophorectomy, hysterectomy).

- Female participants of childbearing potential may be enrolled in the study if the
participant has a negative pregnancy test at Screening (Day 0), on the day of the
first injection (Day 1), on the day of the second injection (Day 29 in Parts 1A and
Part 2, and Day 181 in Part 3); has practiced adequate contraception or has abstained
from all activities that could result in pregnancy for at least 28 days prior to the
first injection (Day 1); and has agreed to continue adequate contraception or
abstinence through 3 months following the second injection (Day 29 in Part 1A and Part
2, and Day 181 in Part 3).

For Part 1B:

- Participants must have been previously enrolled in mRNA-1273-P203 study.

- Female participants of childbearing potential may be enrolled in the study if the
participant has a negative pregnancy test on the day of the first injection
(Open-Label-Day 1) and on the day of the second injection (Open-Label-Day 29).

For Part 1C-1 Homologous Booster Dose:

- Participants must have been previously enrolled in the mRNA-1273-P203 study, are
actively participating in Part 1A or Part 1B and are least 5 months from the last
dose.

- Female participants of childbearing potential may be enrolled in the study if the
participant has a negative pregnancy test on the day of the first injection (BD-Day
1).

Part 1C-2 Heterologous Booster Dose:

- Male or female, 12 to < 18 years of age at the time of consent who, in the opinion of
the investigator, is in good general health based on review of medical history and
screening physical examination AND has completed non-Moderna primary COVID-19
vaccination series under EUA (for example, Pfizer) at least 3 months from consent.

Exclusion Criteria:

For Part 1A, Part 2, and Part 3:

- Has a known history of SARS-CoV-2 infection within 2 weeks prior to administration of
investigational product (IP) or known close contact with anyone with
laboratory-confirmed SARS-CoV-2 infection of COVID-19 within 2 weeks prior to
administration of IP (Part 2 participants only). For Part 3 participants, known
history of SARS-CoV-2 infection within 90 days prior to administration of IP or known
close contact with anyone with laboratory-confirmed SARS-CoV-2 infection or COVID-19
within 90 days prior to administration of IP.

- Travel outside of the United States or home country (Part 2 and Part 3 only) in the 28
days prior to the Screening Visit (Day 0).

- Pregnant or breastfeeding

- Is acutely ill or febrile 24 hours prior to or at the Screening Visit (Day 0). Fever
is defined as a body temperature ≥38.0°Celsius (C)/≥100.4°Farenheit (F). 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.

- Prior administration of an investigational coronavirus (for example, SARS-CoV-2,
SARS-CoV, Middle East Respiratory Syndrome [MERS-CoV]) vaccine

- Current treatment with investigational agents for prophylaxis against COVID-19

- Has a medical, psychiatric, or occupational condition that may pose additional risk as
a result of participation, or that could interfere with safety assessments or
interpretation of results according to the Investigator's judgment

- Current use of any inhaled substance (for example, tobacco or cannabis smoke, nicotine
vapors)

- History of chronic smoking (≥1 cigarette a day) within 1 year of the Screening Visit
(Day 0)

- History of illegal substance use or alcohol abuse within the past 2 years. This
exclusion does not apply to historical cannabis use that was formerly illegal in the
participant's state but is legal at the time of screening.

- History of a diagnosis or condition that, in the judgment of the Investigator, may
affect study endpoint assessment or compromise participant safety, specifically:

- Congenital or acquired immunodeficiency, including human immunodeficiency virus
(HIV) infection

- Suspected active hepatitis

- Has a bleeding disorder that is considered a contraindication to IM injection or
phlebotomy

- Dermatologic conditions that could affect local solicited AR assessments

- History of anaphylaxis, urticaria, or other significant AR requiring medical
intervention after receipt of a vaccine

- Diagnosis of malignancy within the previous 10 years (excluding nonmelanoma skin
cancer)

- Febrile seizures

- Receipt of:

- Any licensed vaccine within 28 days before the first dose of IP or plans for
receipt of any licensed vaccine within 28 days before and/or after each dose of
IP.

- Systemic immunosuppressants or immune-modifying drugs for >14 days in total
within 6 months prior to the day of enrollment (for corticosteroids, ≥20 mg/day
prednisone equivalent). Topical tacrolimus is allowed if not used within 14 days
prior to the day of enrollment. 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.

- Intravenous blood products (red cells, platelets, immunoglobulins) within 3
months prior to enrollment

- Has donated ≥450 milliliters (mL) of blood products within 28 days prior to the
Screening Visit (Day 0) or plans to donate blood products during the study

- Participated in an interventional clinical study within 28 days prior to the Screening
Visit (Day 0) or plans to do so while participating in this study

- Is an immediate family member or has a household contact who is an employee of the
research center or otherwise involved with the conduct of the study

For Part 1C-1 and Part 1C-2:

- Pregnant or breastfeeding.

- Is acutely ill or febrile 24 hours prior to or at the Screening Visit (Day 0). Fever
is defined as a body temperature ≥ 38.0°C/≥ 100.4°F. 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.

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

- History of a diagnosis or condition (after enrolment in Part 1A) that, in the judgment
of the investigator, may affect study endpoint assessment or compromise participant
safety:

- Suspected active hepatitis

- Has a bleeding disorder that is considered a contraindication to IM injection or
phlebotomy

- Dermatologic conditions that could affect local solicited AR assessments

- History of anaphylaxis, urticaria, or other significant AR requiring medical
intervention after receipt of a vaccine

- Diagnosis of malignancy (excluding nonmelanoma skin cancer)

- Receipt of:

• Any authorized or licensed vaccine within 28 days before the first dose of IP or
plans for receipt of any licensed vaccine through 28 days following the last dose of
IP or any seasonal influenza vaccine within 14 days before the first dose of IP or
plans for receipt of any seasonal influenza vaccine 14 days following the last dose of
IP.

- Participated in an interventional clinical study, other than mRNA-1273-P203 study,
within 28 days prior to the Screening Visit (Day 0 [for Part 1C-1], BD-Day 0 [for Part
1C-2]) or plans to do so while participating in this study.

Part 1C-2 Heterologous Booster Dose:

- Has a known history of SARS-CoV-2 infection within 2 weeks prior to administration of
IP or known close contact with anyone with laboratory-confirmed SARS-CoV-2 infection
or COVID 19 within 2 weeks prior to administration of IP.