The purpose of this clinical trial is to learn about the safety and effects of the study vaccine for the potential prevention of influenza. The study vaccine is called Self-Amplifying Ribonucleic Acid vaccine (saRNA vaccine). This study is seeking participants who:

• Are between the age of 18 to 49 years old.
• Are willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.
• Are healthy as determined by medical history, physical examinations, and the study doctor.
• For male participants, can father children and willing to use an acceptable method of contraception. Female participants who are not of childbearing potential; or male participant not able to father children.
• Are capable of giving signed informed consent. Participants will receive either the saRNA vaccine, a licensed Influenza Vaccine (QIV) or a placebo. Participants will not know which vaccine they receive in advance. A placebo does not have any medicine in it but looks just like the study medicine. Participants will receive the study vaccines as a single shot in the arm. We will compare participant experiences to help us determine if the saRNA vaccine is safe and effective. Participants will take part in this study for 6 months. During this time, they will receive the study vaccine and participate in follow-up visits.

Inclusion Criteria:

• Male or female participants 18 to 49 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 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 infection), coronavirus disease 2019 (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 nucleoside-modified messenger ribonucleic acid (modRNA)-platform SARS-CoV-2 vaccine within 60 days of Visit 1
• Previous vaccination with an saRNA or an alphavirus replicon vaccine preparation.
• 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 electrocardiogram (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.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.