This study aims to test a new COVID-19 vaccine in healthy children to see if it is safe and how well it works. The vaccine, called a variant-adapted RNA vaccine, is being given to children in different age groups and with different vaccine histories. There are five smaller studies within this big study, each focusing on different groups of children. Depending on the group, children may get up to four shots of the study vaccine.

• The study is divided into 5 substudies based on age and previous vaccinations.
• Participants will receive 1 to 4 doses of the vaccine depending on their group.
• Eligibility includes being healthy and falling within specific age ranges; those with certain health conditions or past vaccine reactions cannot join.

Children who have had severe reactions to vaccines, certain illnesses, or are taking specific medications cannot participate. The study will help learn more about how the vaccine works in young children and will provide important information for future vaccine use.