"The purpose of this study is to assess the reactogenicity, safety and immune response of GlaxoSmithKlines (GSK) messenger RNA (mRNA)-based multivalent seasonal influenza vaccine candidates administered in healthy younger and older adults."

Inclusion Criteria:

1. Participants, who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of the eDiary, attendance to study contacts and study site visits, and ability to access and utilize a phone or other electronic communications) independently or with the assistance of a caregiver.

2. Written (physically, or digitally) informed consent obtained from the participant prior to performance of any study-specific procedure.

3. A male or female at least 18 YOA at the time of the screening.

4. Healthy participants or medically stable patients as established by medical history, and clinical examination. Participants with chronic medical conditions with or without specific treatment (e.g., chronic metabolic, cardiac, pulmonary, renal, neurologic, and hematologic diseases) are allowed to participate in this study, if considered by the investigator as medically stable. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during 3 months before enrollment.

5. Female participants of non-childbearing potential may be enrolled in the clinical study.

6. Female participants of childbearing potential may be enrolled in the clinical study, if the participant:

   • Has practiced adequate contraception for at least 4 weeks prior to the study intervention administration, and
   • Has a negative urine pregnancy test within 24 hours prior to the study intervention administration, and
   • Has agreed to continue adequate contraception for at least 8 weeks after study intervention administration.

7. Body mass index (BMI) between18 kg/m^² and 33kg/m^².

Exclusion Criteria:

1. Where applicable, FDA toxicity grades will be exclusionary.
2. Immunosuppressive or immunodeficient conditions by history/exam; stable HIV-positive individuals on therapy can participate.
3. History of Guillain-Barré Syndrome within 6 weeks of receiving any vaccine.
4. Significant underlying illness that in the opinion of the investigator would be expected to prevent completion of the study (e.g., life-threatening disease likely to limit survival to less than 1 year).
5. Hypersensitivity to latex.
6. History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s).
7. History of hypersensitivity or severe allergic reaction to any previous influenza vaccine.
8. History of hypersensitivity or severe allergic reaction to any previous mRNA vaccine.
9. History of or current suspicion of myocarditis or pericarditis, or medical issues increasing such risks.
10. Condition that in the judgment of the investigator would make intramuscular injection unsafe.
11. Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the clinical study.
12. History of confirmed influenza infection by local health authority-approved testing methods within 180 days prior to study intervention administration.
13. Use of any investigational or non-registered product (drug, vaccine or invasive medical device) other than the study intervention(s) during the period beginning 28 days before the dose of study intervention(s) (Day -28 to Day 1), or their planned use during the study period.
14. Administration of an influenza vaccine within 180 days before the study intervention administration or planned administration prior to Visit 4 (Day 29) during the study.
15. Administration of any non-live attenuated non-study vaccine in the period starting 14 days before the study intervention administration, or planned administration within 14 days after study intervention administration.
16. Administration of any live attenuated and/or mRNA non-study non-influenza vaccine in the period starting 28 days before the study intervention administration, or planned administration within 28 days after study intervention administration.
17. Chronic administration of immune-modifying drugs.
18. Administration of immunoglobulins and/or any blood products or plasma derivatives during the period starting 90 days before the study intervention, or planned administration during the study period.
19. Administration of anti-tumoral medication during the period starting 90 days before the study intervention or planned administration during the study period.
20. Concurrently participating in another clinical study, at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational intervention.
21. Any study personnel or their immediate dependents, family, or household members.
22. Has a history of chronic alcohol consumption and/or drug abuse as deemed by the investigator to render the potential participant unable/unlikely to provide accurate safety reports or comply with study procedures.
23. Participant is pregnant.
24. Participant is breastfeeding or will (re)start breastfeeding during the study.

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

This is an Observer-blind study.