Flu mRNA-1010 vaccine safety in children aged 9 to under 18

Inclusion Criteria:

* 9 to \<18 years of age inclusive, at the time of signing the informed consent/assent.
* In the Investigator's opinion, the participants/participant's parent(s)/legally authorized representative (LAR(s)) demonstrate adequate understanding of the study requirements and are willing and physically able to comply with protocol mandated follow-up, including all procedures.
* For female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to Day 1, and agreement to continue adequate contraception through 90 days following vaccine administration.

Exclusion Criteria:

* 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; influenza vaccine; or components of influenza vaccine, including egg protein.
* Acutely ill or febrile (temperature ≥38.0 degrees Celsius (℃) \[100.4 degrees Fahrenheit \[°F\]) within 72 hours prior to or at the Screening Visit or Day 1. Participants who are acutely ill or febrile may be scheduled for a separate dosing visit within the window specified for Screening.
* History of a diagnosis or condition not specifically listed as exclusionary that, in the judgment of the Investigator, is clinically unstable, potentially affects participant safety, the assessment of study endpoints, the assessment of immune response, or adherence to study procedures.
* Any prior systemic immunosuppressants, immunomodulators (for example, methotrexate, cyclosporine), or long-acting biological therapies that may affect immune responses (for example, dupilumab, omalizumab). Short courses (\<7 days) of oral corticosteroids are allowed if completed at least 3 months prior to Day 1. Inhaled, nasal, topical steroids, and subcutaneous injected allergy immunotherapy (allergy shots) are allowed.
* Received systemic immunoglobulins or blood products within 90 days prior to the Screening Visit or plans to receive them during the study.
* Received any nonstudy vaccine authorized or approved by local health agency within 14 days (28 days for live vaccines and mRNA vaccines) prior to Day 1 (inclusive) or plans to receive any nonstudy vaccine within 28 days post study vaccine administration.
* Donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit or plans to donate blood products within 28 days post study vaccine administration.

Note: Other protocol-defined inclusion and exclusion criteria may apply.