Study of mRNA-1010 Seasonal Influenza Vaccine in Adults

Inclusion Criteria:

• Investigator has assessed that the participant understands and is willing andphysically able to comply with protocol mandated follow up, including allprocedures.

• For assigned females at birth and of childbearing potential: negative pregnancytest, adequate contraception or has abstained from all activities that could resultin pregnancy for at least 28 days prior to Day 1, and agreement to continue adequatecontraception through 90 days following vaccine administration.

Part A:

• At least 18 years of age inclusive, at the time of signing the informed consent form (ICF).

Part B:

• At least 18 and <65 years of age, at the time of signing the ICF.

Part C:

• At least 65 years of age or older, at the time of signing the ICF.

Exclusion Criteria:

• Participant has had close contact with someone with laboratory-confirmed influenzainfection or with someone who has been treated with antiviral therapies forinfluenza (for example, Tamiflu®) within the past 5 days prior to Day 1.

• Participant is acutely ill or febrile (temperature ≥38.0℃elcius [100.4°Fahrenheit]) 72 hours prior to or at the Screening visit or Day 1. Participants meeting thiscriterion may be rescheduled within the 28-day screening window.

• Participant has a history of a diagnosis or condition that, in the judgment of theinvestigator, is clinically unstable or may affect participant safety, assessment ofsafety endpoints, assessment of immune response, or adherence to study procedures.

• Reported history of congenital or acquired immunodeficiency, immunosuppressivecondition or immune-mediated disease, asplenia, or recurrent severe infections.

• Participant has tested positive for influenza by local health authority-approvedtesting methods within 150 days (for Part A) or 180 days (for Parts B and C) priorto Day 1.

• Reported history of anaphylaxis or severe hypersensitivity reaction after receipt ofmRNA vaccines or any components of the mRNA-1010 or influenza vaccines, includingegg protein.

• Participant has received systemic immunosuppressants for >14 days in total within 180 days prior to Day 1 (for corticosteroids, ≥10 mg/day of prednisone orequivalent) or is anticipating the need for systemic immunosuppressive treatment atany time during participation in the study. Inhaled, nasal and topical steroids areallowed. Intra-articular and epidural steroid injections are not allowed within 28days before and/or after study intervention dosing.

• Participant has received any vaccine authorized or approved by local health agency ≤28 days prior to study intervention dosing (Day 1) or plans to receive a vaccineauthorized or approved by local health agency within 28 days before or after studyintervention dosing.

• Participant has received a licensed seasonal influenza vaccine within 5 months (150days) (for part A) or within 6 months (180 days) (for Parts B and C) prior to Day 1.

• Participant has participated in any investigational seasonal influenza vaccine studywithin12 months prior to Day 1.

• Participant is not aware whether they have received an influenza vaccine in the mostrecent influenza season (in the prior 12 months) (for Part A) or since September 2022 (for Parts B and C).

• Participant has donated ≥450 milliliters (mL) of blood products within 28 days priorto Day 1 or plans to donate blood products during the study.

Note: Other inclusion and exclusion criteria may apply.