A Study to Evaluate the Safety, Tolerability, and Immunogenicity of a Modified RNA Vaccine Against Influenza

Substudy A

Inclusion Criteria

• Male or female participants 65 to 85 years of age.
• Participants who are willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.
• Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study.
• Male participant who is able to father children and willing to use an acceptable method of contraception; or female participant not of childbearing potential; or male participant not able to father children.
• Capable of giving signed informed consent.

Exclusion Criteria

• Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
• History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis) to any component of the study intervention.
• Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.
• Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.
• Women who are pregnant or breastfeeding.
• Allergy to egg proteins (egg or egg products) or chicken proteins.
• Participant who has had significant exposure to laboratory-confirmed SARS-CoV-2 infection, COVID-19, or influenza in the past 14 days known prior to Visit 1
• Any participant who has a SARS-CoV-2 RT-PCR or antigen test in the past 10 days prior to Visit 1 that has not been confirmed as negative.
• Individuals who receive treatment with radiotherapy or immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids, eg, for cancer or an autoimmune disease, or planned receipt throughout the study.
• Receipt of blood/plasma products, immunoglobulin, or monoclonal antibodies, from 60 days before study intervention administration, or planned receipt throughout the study.
• Vaccination with any influenza vaccine within 6 months (175 days) before study intervention administration.
• Any participant who has received or plans to receive a modRNA-platform SARS-CoV-2 vaccine within 60 days of Visit 1
• Participation in other studies involving study intervention within 28 days prior to study entry and/or during study participation.
• Screening hematology/blood chemistry lab >=Grade 1 abnormality. Except Bilirubin, other stable Grade1 abnormalities may be considered eligible by Investigator.
• Screening ECG that is consistent with probable or possible myocarditis or pericarditis, or demonstrates clinically relevant abnormalities that may affect participant safety or study results.
• Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.
• Participation in strenuous or endurance exercise through Visit 3.
• Prior history of heart disease.

Substudy B

Inclusion Criteria

• Male or female participants 65 to 85 years of age or .Male or female participants 18 to 64 years of age
• Participants who are willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.
• Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study.
• For participants 65 to 85 years of age at the time of enrollment, receipt of licensed influenza vaccination for the 2021-2022 northern hemisphere season >4 months (120 days) before study intervention administration.
• Capable of giving signed informed consen