Participants will be enrolled into 1 of 2 age cohorts: Cohort A for adults ≥65 to <80 years of age or Cohort B for adults ≥18 to <65 year of age. In Cohort A, approximately 600 participants will be randomized (in equal allocation ratio) into the investigational treatment arms, stratified by influenza vaccine status in the most recent influenza season (received or not received since September 2022). In Cohort B, approximately 624 participants will be randomized (in equal allocation ratio) into the investigational treatment arms, stratified by 2 age groups: ≥18 to <50 years and ≥50 to <65 years of age and by influenza vaccine status in the most recent influenza season (received or not received since September 2022).
The purpose of this study is to generate sufficient safety, reactogenicity, and immunogenicity data to enable selection of an mRNA-1083 vaccine composition and dose level to evaluate in a subsequent Phase 3 clinical trial in adults.