A Study of Updated mRNA Seasonal Influenza Vaccines

Key Inclusion Criteria:

* Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol.
* Female participants must be of non-childbearing potential.

Key Exclusion Criteria:

* Acutely ill or febrile (temperature ≥38.0 degrees Celsius (℃) \[100.4° Fahrenheit \[F\]\]) within 72 hours prior to at the Screening Visit or Day 1.
* History of anaphylaxis or severe hypersensitivity reaction requiring medical intervention after receipt of any mRNA vaccine or therapeutic or any components of an mRNA vaccine or therapeutic
* History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable, potentially affects participant safety, could interfere with interpretation of study results or limit the participant's participation in the study.
* History of myocarditis, pericarditis, or myopericarditis within 90 days prior to Day 1 (Baseline).
* Received corticosteroids at ≥10 mg/day of prednisone or equivalent for \>14 days in total within 90 days prior to Day 1 (Baseline) or within 28 days after Day 1.
* Received systemic immunosuppressive treatment, including long-acting biological therapies that affect immune responses (eg, infliximab), within 180 days prior to Day 1 (Baseline) or plans to do so during the study.
* Tested positive for influenza by US Centers for Disease Control and Prevention (CDC)-recommended testing methods within 180 days prior to Day 1.
* Received systemic immunoglobulins or blood products within 90 days prior to Day 1 (Baseline) or plans to do so during the study.
* Received any vaccine (excluding seasonal influenza vaccine) authorized or approved by local health agency within 28 days prior to Day 1 (Baseline) or plans to do so within 14 days after Day 1 (Baseline).
* Received a licensed or investigational seasonal influenza vaccine within 6 months prior to Day 1 (Baseline) or plans to do so (outside of this study) at any time during the study.