The aim of this study is to evaluate the safety, reactogenicity, and immune response of the different formulations of the investigational RSV/hMPV combination vaccine and investigational hMPV vaccine in younger and older adults.

Inclusion Criteria:

Participants are eligible to be included in the study only if all of the following criteria apply:

• Written informed consent obtained from the participant prior to performance of any study-specific procedure.
• Participants who can and will comply with the requirements of the protocol (e.g., completion of the eDiary, return for follow-up visits, ability to access and utilize a phone or other electronic communications).
• Note: For OA participants, in case of physical incapacity that would preclude the self-completion of the eDiaries, either site staff can assist the participant (for activities performed during site visits) and/or the participant may assign a caregiver to assist him/her with this activity (for activities performed at home). However, at no time will the site staff or caregiver evaluate the participant's health status while answering eDiaries or make decisions on behalf of the participant.
• Body Mass Index (BMI) between 18 kg/m^2 and 33 kg/m^2, inclusive.

Specific inclusion criteria for OA

• A male or female between and including, 60 to 80 YOA at the time of the study intervention administration.
• Healthy participants or medically stable patients as established by medical history, physical examination (and normal screening laboratory tests including Grade 1 laboratory abnormalities that are not-clinically significant in Phase 1 only).

Specific inclusion criteria for YA

• A male or female participant between and including 18 to 49 YOA at the time of the study intervention administration.

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

• Participants of non-childbearing potential may be enrolled in the clinical study.

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

  • has used two methods of contraception, at-least one of which must be a highly effective method, and the other being male condom for male sexual partners of POCBP to be used during sexual intercourse (with the exception of sexual abstinence, vasectomized partner and male partner who has been sterilized, in which case contraception is not required), at-least 30 days prior to study intervention administration, and has agreed to continue using above contraception requirements for 8 weeks after study intervention administration.
  • has a negative serum pregnancy test at screening and negative urine pregnancy test prior to study intervention administration on Day 1.

Exclusion Criteria:

Participants are excluded from participating in the study if any of the following criteria apply:

• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention(s).

• Any medical condition that in the judgment of the investigator would make IM injection unsafe.

• Any confirmed or suspected immunosuppressive or immunodeficient condition resulting from disease (e.g., current malignancy, HIV) or immunosuppressive/cytotoxic therapy (e.g., medication used during cancer chemotherapy, organ transplantation, or to treat autoimmune disorders), based on medical history and physical examination (no laboratory testing required).

• Acute or unstable chronic pulmonary, cardiovascular, hepatic, or renal functional abnormality, as determined by physical examination and medical history.

• Documented history of HIV, HBV, HCV infection.

• History of RSV and /or hMPV-associated illness, diagnosed serologically or microbiologically in the last 12 months.

• Recurrent history or uncontrolled neurological disorders or seizures, or history of demyelinating conditions (including GBS).

• Any history of dementia or any medical condition that moderately or severely impairs cognition.

• 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.

• Significant underlying illness that in the opinion of the investigator would be expected to prevent completion of the study (e.g., life-threatening disease).

• 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 study intervention administration (Day -29 to Day 1), or within 5 half-lives, whichever is longer, or their planned use during the study period.

• Has previously received an investigational or approved vaccine or antibody for prevention of hMPV and/or RSV-associated diseases.

• 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.

  • Up to 3 months prior to the study intervention administration:

    • For corticosteroids, this will mean prednisone equivalent >=20 mg/day for adult participants. Inhaled, topical and intra-articular steroids are allowed.
    • Administration of immunoglobulins and/or any blood products or plasma derivatives.

  • Up to 6 months prior to study intervention administration: long-acting immune-modifying drugs including among others immunotherapy (e.g., TNF-inhibitors), monoclonal antibodies, antitumoral medication.

• 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.

• 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.

• Participation of any study personnel or their immediate dependents, family, or household members.

• Planned move during the study period that will prohibit participating in the study until study end.

• Bedridden participants.

Specific exclusion criteria for OA population

• Planned or actual administration of a vaccine not foreseen by the study protocol in the period beginning 30 days before study intervention administration (Day - 29 to Day 1), or their planned use 30 days after study intervention administration, with the exception of inactivated, subunit and split influenza vaccines or COVID-19 vaccines which can be administered up to 14 days before or from 14 days after the study intervention administration.

• At screening (Phase 1 only): Any laboratory abnormality. Grade 1 laboratory abnormalities that are not-clinically significant* may be included in the study.

  • The investigators should use their clinical judgment to decide which abnormalities are clinically significant.

Specific exclusion criteria for YA population

• Pregnant or lactating participant.
• Female planning to become pregnant or planning to discontinue contraceptive precautions for 8 weeks after study intervention administration.
• Planned or actual administration of a vaccine not foreseen by the study protocol in the period beginning 30 days before study intervention administration (Day - 29 to Day 1), or their planned use 30 days after study intervention administration.
• At screening: Any hematologic and/or biochemical laboratory abnormality.
• Use of any medication, including prescription and non-prescription drugs or herbal remedies (such as St. John's wort) beginning 14 days (or 5 half-lives) prior to study intervention administration and/or planned administration during the study period. Certain medications may be permitted (Eg: contraceptives for POCBP).

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/gsk-patient-level-data-sharing-july2025.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.