This study is testing the safety of two special vaccines called mRNA-1273.214 and mRNA-1273.815 for young children. These vaccines are designed to help the body fight off a virus called SARS-CoV-2, which causes COVID-19. Children aged 6 months to less than 6 years will receive these vaccines in different parts of the study. Some children will get two doses if they have never been vaccinated, while others who already had two doses will receive a booster shot. The study checks if the vaccines are safe and how well they help the body make defenses, called antibodies.

• **Eligibility:** Participants can be ages 6 months to less than 6 years, with special rules for different parts of the study.
• **Study Visits:** The study involves multiple visits, and children must be healthy or have stable conditions.
• **Exclusions:** Children with recent COVID-19, allergies to vaccine ingredients, or certain medical conditions cannot join.