The purpose of this study is to assess the reactogenicity, safety and immune response of various formulations of the RSV mRNA investigational vaccine administered in healthy participants 18-45 years of age.

Inclusion Criteria:

• Participants, who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of the eDiary, return for follow-up visits).

• Written informed consent obtained from the participant prior to performance of any study-specific procedure.

• Healthy participants as established by medical history, clinical examination and laboratory assessment at screening.

• Male or female between and including 18 and 45 years of age at the time of enrollment into the study.

• Body mass index more than or equal to (>=) 18 kg/m^2 and less than (<) 40 kg/m^2.

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

• Female participants of childbearing potential may be enrolled in the study if the participant:

  • has practiced adequate contraception for 1 month prior to study intervention administration period, and
  • has a negative pregnancy test (on urine sample) on the day of study intervention administration, and
  • has agreed to continue adequate contraception during the entire treatment period and for at least 1 month after completion of the study intervention administration series.

Exclusion Criteria:

Medical conditions

• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study interventions.
• Hypersensitivity to latex.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination (no laboratory testing required).
• Acute or chronic clinically significant pulmonary, cardiovascular, hepatic or renal functional abnormality.
• Recurrent history or uncontrolled neurological disorders or seizures.
• Documented HIV, HBV, or HCV-positive participant.
• Lymphoproliferative disorder or malignancy within 5 years before the first dose of study intervention administration.
• History of or current suspicion of myocarditis or pericarditis.

Prior/Concomitant therapy

• Use of any investigational or non-registered product (drug, vaccine, or invasive medical device) other than the study intervention during the period beginning 30 days before the first dose of study intervention administration (Day -29 to Day 1), or their planned use during the study period.
• Has previously received an investigational or approved vaccine or antibody for prevention of RSV infection.
• Planned administration/administration of a vaccine in the period starting 30 days before the first dose and ending 30 days after the last dose of study intervention administration, except for inactivated vaccines for influenza if they are received at least 14 days before the first dose or 14 days after the last study intervention administration.
• Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune-modifying treatments at any time up to the end of the study.

Prior/Concurrent clinical study experience

• 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 (drug/invasive medical device).

Other exclusion criteria

• Pregnant or lactating female participant.
• Female participant planning to become pregnant or planning to discontinue contraceptive precautions within 1 month following the last study intervention administration.
• Alcoholism or substance use disorder within the past 24 months.
• Any study personnel or their immediate dependents, family, or household members.
• Participants with extensive body markings or conditions in the deltoid region that may preclude accurate assessment of local reactogenicity.

GSK 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 terminated asset(s) 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.

Data will be collected in an observer-blind manner