Seasonal influenza causes considerable morbidity and mortality, with influenza A and B viruses driving most influenza-associated hospitalizations and deaths. Vaccination remains a key influenza prevention strategy; however, current seasonal influenza vaccines based on traditional platforms provide inconsistent protection. Messenger RNA - based vaccines may offer several key advantages over other vaccines, including the flexibility to optimize antigen expression to enhance immunogenicity without the need for adjuvants. mRNA-1010 is an investigational seasonal influenza vaccine candidate that encodes hemagglutinins (HAs) from WHO-recommended influenza strains. In this randomized, phase 2 study, safety, reactogenicity, and immunogenicity of 3 mRNA-1010 vaccine candidate compositions were evaluated in healthy adults aged 18-49 y in the United States (NCT05868382). Eligible participants were randomly assigned to receive a single dose of one of the mRNA-1010 compositions at several dose levels. The primary objective was the safety and reactogenicity of mRNA-1010 vaccine candidate compositions against vaccine-matched strains; secondary and exploratory objectives included humoral and cellular immunogenicity at evaluable timepoints, respectively. Two hundred and seventy participants received study vaccination between May and December 2023. All mRNA-1010 vaccine compositions were well tolerated, with no safety concerns identified; all compositions induced HA-specific humoral and cellular immune responses against all influenza strains evaluated, with the optimized compositions exhibiting higher immune responses against influenza B strains compared with the original mRNA-1010 composition. These results, together with findings from other mRNA-1010 clinical studies, support continued evaluation of mRNA-1010 for enhanced protection against seasonal influenza.