Dose-Confirmation Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of mRNA-1273 COVID-19 Vaccine in Adults Aged 18 Years and Older

Key Inclusion Criteria

Each participant must meet all of the following criteria during the screening period and at Day 1, unless noted otherwise, to be enrolled in this study:

• Male or female, 18 years of age or older at the time of consent (Screening Visit, Day 0). For Part B, participants must have been previously enrolled in the mRNA-1273 P201 study.
• Understands and agrees to comply with the study procedures and provides written informed consent.
• According to the assessment of the investigator, is in good general health and can comply with study procedures.
• Female participants of nonchildbearing potential may be enrolled in the study. Nonchildbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to Screening (Day 0) without an alternative medical cause). A follicle-stimulating hormone (FSH) level may be measured at the discretion of the investigator to confirm postmenopausal status.
• 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 Screening (Day 0) and on the day of the first injection (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 injection (Day 1).
• Has agreed to continue adequate contraception through 3 months following the second injection (Day 29).
• Is not currently breastfeeding.

Adequate female contraception is defined as consistent and correct use of a Food and Drug Administration (FDA) approved contraceptive method in accordance with the product label. For example:

• Barrier method (such as condoms, diaphragm, or cervical cap) used in conjunction with spermicide
• Intrauterine device
• Prescription hormonal contraceptive taken or administered via oral (pill), transdermal (patch), subdermal, or IM route
• Sterilization of a female participant's monogamous male partner prior to entry into the study Note: periodic abstinence (for example, calendar, ovulation, symptothermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception.
• Male participants engaging in activity that could result in pregnancy of sexual partners must agree to practice adequate contraception from the time of the first injection and through 3 months after the last injection.

Adequate contraception for male participants is defined as:

• Monogamous relationship with a female partner using an intrauterine device or hormonal contraception (described above)
• Use of barrier methods and spermicide
• History of surgical sterilization
• Male participants with partners who have become pregnant prior to Screening are eligible to participate in the study.

Additional Key Inclusion Criteria for Part C

• Participants must have been previously enrolled in the mRNA-1273-P301 study and must have received 2 doses of mRNA-1273 in Part A, has been unblinded and aware of their actual treatment in Study mRNA-1273-P301, must have been compliant in Study mRNA-1273-P301 (was not withdrawn or discontinued early), and has been at least 6 months since their second dose in Study mRNA-1273-P301 prior to enrollment in this part.

Key Exclusion Criteria

Participants meeting any of the following criteria at the Screening Visit (Day 0) or at Day 1, unless noted otherwise, will be excluded from the study:

• 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/100.4°Fahrenheit. Participants meeting this criterion may be rescheduled within the relevant window periods. Afebrile participants with minor illnesses can be enrolled at the discretion of the investigator.
• Current treatment with investigational agents for prophylaxis against COVID-19.
• Has