This study is about testing a new version of the mRNA-1283 COVID-19 vaccine to see how well it works and if it's safe. Immunogenicity means how well the vaccine triggers an immune response to fight the virus. Participants will get a new formula of the vaccine, called mNEXSPIKE®, designed to target different virus variants. The study is divided into parts, and Part B focuses on collecting more safety data. People aged 65 and older, and those aged 12 to under 65 with health risks, can join.

• Participants must attend multiple visits for follow-up and data collection.
• Those taking part should not have had a COVID-19 infection or vaccine in the last 6 months.
• Pregnant participants must have a negative test and use contraception during the study.

Before joining, participants will discuss with an investigator to ensure they understand the procedures and are able to follow the study requirements. Those who are sick or have had certain treatments recently may not be eligible to join.