A Study to Investigate the Immunogenicity, Reactogenicity, and Safety of mRNA-1083 (Influenza and COVID-19) Vaccine in Adults ≥18 to <65 Years of Age

Inclusion Criteria: * Medically stable. * Participants who could become pregnant: negative pregnancy test and contraception for at least 28 days prior to Day 1 and for at least 90 days after the study intervention administration. * Have received ≥2 coronavirus disease 2019 (COVID-19) vaccines, with the last COVID-19 vaccine administered \>150 days prior to Day 1. Exclusion Criteria: * History of a diagnosis or condition that is clinically unstable or may affect participant safety, assessment of study endpoints, assessment of immune response, or adherence to study procedures. * Tested positive for influenza by local health authority-approved testing methods within 150 days prior to Day 1. * History of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection within 150 days prior to Day 1. * Received systemic immunosuppressive treatment, including long-acting biological therapies that affect immune responses (eg, infliximab), within 180 days prior to Day 1 or plans to do so during the study. * Received or plans to receive any vaccine authorized or approved by a local health agency within 28 days prior to Day 1 or plans to do so within 28 days post study injection. * Received a licensed seasonal influenza vaccine within 150 days prior to Day 1. * Received a licensed/authorized COVID-19 vaccine within 150 days prior to Day 1. * Received any investigational influenza vaccine, investigational COVID-19 vaccine, or investigational combination vaccine for influenza and COVID-19 within 12 months prior to Day 1. * Participated in a clinical study with investigational treatment within 90 days prior to Day 1 (Baseline). Other protocol-defined inclusion/exclusion criteria may apply.