The purpose of this Phase 1/2 study is to generate sufficient safety and immunogenicity data of mRNA-1018 pandemic influenza candidate vaccines in healthy adults ≥18 years of age to enable the initiation of a large Phase 3 trial with one selected vaccine candidate. The study will be conducted in 2 Parts (Part A and Part B) that will enroll and run concurrently. Part A of the study will evaluate 4 vaccine candidates (H5N8, H7N9, H5 only, and H7 only). Part B of the study will evaluate a single vaccine candidate (H5 only-CG).

Key Inclusion Criteria:

• Healthy as determined by medical evaluation including medical history, physical examination, laboratory tests, and results of electrocardiogram testing.
• Body mass index of 18 kilograms (kg)/square meter (m^2) to 39 kg/m^2 (inclusive) at the Screening visit.
• For female participants of childbearing potential: negative pregnancy test, adequate contraception, and not currently breastfeeding.

Key Exclusion Criteria:

• Participant is acutely ill or febrile (body temperature ≥ 38.0 degrees Celsius [°C]/100.4 degrees Fahrenheit [°F]) 72 hours prior to or at the Screening Visit or Day 1.
• History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures.
• Any medical, psychiatric, or occupational condition, including reported history of substance abuse, that, in the opinion of the Investigator, might pose additional risk due to participation in the study or could interfere with the interpretation of study results.
• Participant has received systemic immunosuppressants or immune-modifying drugs for >14 days in total within 180 days prior to Screening Visit (for corticosteroids ≥10 milligrams [mg]/day of prednisone equivalent) or is anticipating the need for immunosuppressive treatment at any time during participation in the study.
• Participant has received any vaccine authorized or approved by local health agency including mRNA vaccine ≤28 days prior to study intervention (Day 1) or plans to receive a vaccine authorized or approved by local health agency within 28 days before or after the study intervention.
• Participant has participated in an interventional clinical study within 28 days prior to the Screening visit based on the medical history interview or plans to do so while participating in this study.

Other inclusion/exclusion criteria may apply.