A Study to Investigate Multiple Sclerosis Relapse Prevention With mRNA-1195 Compared With Placebo in Participants Aged 18 to ≤55 Years

Inclusion Criteria: * Medically stable as determined by Investigator's medical evaluation, which will include assessment of medical history, physical examination, laboratory testing, and review of any previously conducted cardiac monitoring. * Participants who are Epstein-Barr virus (EBV)-seropositive at screening. * Participants diagnosed with relapsing multiple sclerosis, including those with a single clinical attack (that is, clinically isolated syndrome \[CIS\]), as well as participants diagnosed with radiologically isolated syndrome, within 24 months of Screening Visit (that is, early in their multiple sclerosis course) and in the opinion of the Investigator have been neurologically stable for at least 30 days prior to Visit 1/Day 1. * A participant who could become pregnant is eligible to participate if they are not pregnant or breast/chest feeding and using a highly effective contraceptive method. Exclusion Criteria: * Acutely ill or febrile (temperature ≥38.0 degrees Celsius (℃) \[100.4 Fahrenheit °F\]) within 72 hours prior to or at screening or Day 1. * History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of study endpoints, assessment of immune response, or adherence to study procedures. * Received or plans to receive any non-study vaccine (including authorized or approved vaccines for the prevention of coronavirus disease 2019 \[COVID-19\] regardless of vaccine type) w…