Study Overview: This study is testing a new vaccine, called mRNA-1195, to see if it's safe and causes any reactions. It targets the Epstein-Barr Virus (EBV), which can make people sick. The study involves two parts, starting with adults aged 18 to 55, and then focusing on younger adults aged 18 to 30. Participants must be healthy and, for safety, women who can have children need a negative pregnancy test and must use birth control during the study.

Key Points:

• The study involves multiple visits and check-ups to ensure participant safety.
• Participants must not have recent illnesses or certain medical conditions.
• Participants cannot have had certain vaccines or donated a lot of blood recently.

Eligibility: To join, participants need to be generally healthy and either have had EBV before or not (depending on the study part). They must not be sick with a fever or have unstable health conditions. They should not have a history of heart-related inflammation or be on certain medications that affect the immune system.