The purpose of this study is to evaluate humoral immunogenicity after 2 doses of mRNA-1018-H5, and to evaluate the safety and reactogenicity of mRNA-1018-H5 in adults ≥18 years of age.

Key Inclusion Criteria:

• Healthy as determined by medical evaluation including medical history; and physical examination. Participants with clinically stable chronic medical conditions are permitted.

• Participants who are assigned female at birth or could become pregnant are eligible to participate if the participant is not pregnant or breast/chest feeding, and one of the following conditions applies:

  • Is a person of nonchildbearing potential (PONCBP) OR
  • Is a person of childbearing potential (POCBP)
  • A POCBP must have a negative highly sensitive pregnancy test at Screening and on the day of the first dose of study intervention.

Key Exclusion Criteria:

• Participant is acutely ill or febrile (body temperature ≥ 38.0 degrees Celsius [°C]/100.4 degrees Fahrenheit [°F]) within 72 hours prior to or at the Screening Visit or Day 1.
• History of myocarditis, pericarditis, or myopericarditis.
• History of Guillain-Barre syndrome.
• Reported history of congenital or acquired immunodeficiency, immunosuppressive condition, asplenia, or recurrent severe infections disease.
• Treated with antiviral therapies for influenza (eg, Tamiflu, Xofluza) within 28 days prior to Day 1.
• Prior receipt of a pandemic influenza vaccine or participation in any pandemic influenza vaccine clinical study, including the mRNA-1018-P101 study.
• Any medical, psychiatric, or occupational condition, that, in the opinion of the Investigator, might pose additional risk due to participation in the study or could interfere with adherence to study procedures or the interpretation of study results.
• Participant has received systemic immunosuppressants including long-acting biological therapies that affect immune responses (eg, infliximab, methotrexate, omalizumab, etc.), within 180 days prior to Screening or plans to do so at any time during participation in the study.
• Participant has received corticosteroids at ≥10 mg/day of prednisone or equivalent for >14 days in total within 90 days prior to Day 1 (Baseline) or is anticipating the need for corticosteroids at any time during the study.
• Participants has received any licensed 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 21 days after the study intervention.
• Participant has participated in an interventional clinical study within 90 days prior to the Screening visit based on the medical history interview or plans to do so while participating in this study.

Note: Other inclusion/exclusion criteria may apply.