The 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 three 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 a Sentinel Cohort A paired with a Main Cohort, evaluating the safety and efficacy of the 2-administration regimen and a Sentinel Cohort B, evaluating the safety of the 3- administration regimen. The Sentinel Cohorts will assess the safety of the dose levels and regimens in a stepwise manner. If the participants consent to the LTE, they will be followed up for an additional 30 months after the last planned visit in the Core Study, to assess the long-term effects of the vaccine.