A Study to Evaluate Efficacy, Safety, and Immunogenicity of mRNA-1273 Vaccine in Adults Aged 18 Years and Older to Prevent COVID-19

The mRNA-1273 vaccine is being developed to prevent COVID-19, the disease resulting from Severe Acute Respiratory Syndrome coronavirus (SARS-CoV-2) infection. The study is designed to primarily evaluate the efficacy, safety, and immunogenicity of mRNA-1273 to prevent COVID-19 for up to 2 years after the second dose of mRNA-1273.

This is a 3-part Phase 3 study, with Part A (Blinded Phase), Part B (Open-label Observational Phase), and Part C (Booster Dose Phase). Participants in Part A are blinded to their treatment assignment, with participants receiving either mRNA-1273 vaccine or placebo. Part B of the study is designed to offer participants to be unblinded so that participants who received placebo in Part A can request 2 doses of open-label mRNA-1273 vaccine. Additionally, participants who choose to be unblinded and were only able to receive 1 dose of mRNA-1273 due to administrative reasons, can choose to receive the second dose of mRNA-1273 during Part B. In Part C, a booster dose will be provided for all eligible participants who choose to receive one.

Please access www.modernatx.com/cove-study for additional information, such as Study Overview, Participation, and Site Locations along with contact numbers for each location for the study.

Eligibility Criteria:
Inclusion Criteria:

* (Part A only) Participants who are at high risk of SARS-CoV-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to SARS-CoV-2 and COVID-19.
* Understands and agrees to comply with the study procedures and provides written informed consent.
* Able to comply with study procedures based on the assessment of the Investigator.
* Female participants of non-childbearing potential may be enrolled in the study. Non-childbearing 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 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 and on the day of the first dose (Day 1, open-label Day 1, and booster dose 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 (Day 1).
  * Has agreed to continue adequate contraception through 3 months following the last dose (Day 29, open-label Day 29, and booster dose Day 1).
  * Is not currently breastfeeding.
* Healthy adults or adults with pre-existing medical conditions who are in stable condition. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment.
* (Part C Only) Is currently enrolled in Part B of the current study (mRNA-1273-P301).
* (Part C Only) Has received at least 1 dose of mRNA-1273 in the current study (mRNA-1273-P301).

Exclusion Criteria:

* Is acutely ill or febrile 72 hours prior to or at Screening or dosing (Part B and Part C). 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/dosed at the discretion of the Investigator.
* Is pregnant or breastfeeding.
* (Part A Only) Known history of SARS-CoV-2 infection.
* Prior (Part A) or concurrent (Part B and Part C) administration of non-study coronavirus (SARS-CoV, Middle East Respiratory Syndrome \[MERS\]-CoV) vaccine or current/planned simultaneous participation in another interventional study to prevent or treat COVID-19.
* (Part A Only) Demonstrated inability to comply with the study procedures.
* An immediate family member or household member of this study's personnel.
* Known or suspected allergy or history of anaphylaxis, urticaria, or other significant adverse reaction to the vaccine or its excipients.
* Bleeding disorder considered a contraindication to intramuscular injection or phlebotomy.
* Has received or plans to receive a vaccine within 28 days prior to the first dose (Day 1) or plans to receive a non-study vaccine within 28 days prior to or after any dose of investigational product (IP) (except for seasonal influenza vaccine).
* (Part A only) Has participated in an interventional clinical study within 28 days prior to the day of enrollment.
* Immunosuppressive or immunodeficient state, including human immunodeficiency virus (HIV) infection, asplenia, and recurrent severe infections.
* Has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to IP dose administration (for corticosteroids ≥20 milligram (mg)/day of prednisone equivalent).
* Has received systemic immunoglobulins or blood products within 3 months prior to the day of IP dose administration.
* Has donated ≥450 milliliters (mL) of blood products within 28 days prior to IP dose administration.