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

This is a 3-part Phase 2a study, with Part A (Blinded Phase), Part B (Open-label Interventional Phase), and Part C (Rollover Proof of Concept). Participants in Part A are blinded to their treatment assignment, with participants receiving either 2 active mRNA-1273 vaccine doses 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 originally received 1 or 2 doses of mRNA-1273 (50 microgram \[μg\] or 100 μg vaccine) during Part A, will have the opportunity to request to receive a single booster dose of mRNA-1273. Part C will be a proof-of-concept rollover study to evaluate a vaccine to treat mutations of SARS-CoV2, such as the S-protein of the B.1.351 variant. Part C will include approximately 60 participants, who are currently enrolled in Moderna's Phase 3 mRNA-1273-P301 study (NCT04470427), have already been unblinded, and have previously received 2 doses of mRNA-1273 at least 6 months earlier. At enrollment into Part C of this study, their participation in mRNA-1273-P301 study will be terminated. Part C will evaluate the safety and immunogenicity of 2 dose levels (20 µg and 50 µg) of mRNA-1273.351 and mRNA-1273/mRNA-1273.351 mixture (50 µg total), given as a single booster dose.