RecruitingBiological interventionStudy to Evaluate Safety, Efficacy and Immunogenicity of Acne mRNA Vaccine in Adults With Moderate to Severe AcneThe purpose of the trial is to evaluate the safety, efficacy and immunogenicity of up to 3 intramuscular injections of the Acne mRNA vaccine candidate at up to four dose levels in adult participants aged 18 to 45 years with moderate to severe acne. This trial will consist of a Core Study followed by an optional Long-Term Extension (LTE). The Core Study will consist of: * Two cohorts evaluating the 2-administration regimen (Cohorts A): Sentinel Cohort A and Main Cohort A. * Two cohorts evaluating the 3-administration regimen (Cohorts B): Sentinel Cohort B and Main Cohort B. Participants from Sentinel Cohorts A and B and from Main Cohort A will be invited to an additional 30-month follow up after completing their last planned Core Study visit to assess the long-term effects of the vaccine. Participants from Main Cohort B will be invited to another LTE study managed through a separate protocol.