This study is testing a new vaccine called mRNA-1647 for a virus called cytomegalovirus (CMV). CMV is a common virus that can infect anyone. The study is looking at how safe the vaccine is and how well it helps the body fight off the virus. The study will have two parts: one to find the right dose and another to check safety.

Participants will be healthy boys and girls aged 9 to 15, and young women aged 16 to 25. People can join if they are in good health and fit certain health checks, like not having certain liver or blood problems. They also should not have had other vaccines or treatments recently.

• The study will take place in two parts to check different doses and safety.
• Participants will need to attend several visits for tests and vaccine shots.
• Those joining should be in good health and not have certain medical conditions or treatments.