The goal of this clinical trial is to quantify the immune response in older Korean adults after a RSVpreF vaccination. It will also learn about the safety and tolerability of RSVpreF vaccination. The main questions it aims to answer are:

What local reactions and systemic events do participants have after a RSVpreF vaccination? What medical problems do participants have after a RSVpreF vaccination? Researchers will compare RSVpreF to a placebo (a look-alike substance that contains no RSVpreF) to see if RSVpreF is safe and well tolerated. It will also examine the change in antibody levels (immune responses) before and after vaccination.

Participants will:

Receive the RSVpreF vaccination or a placebo injection once at Visit 1. Visit the clinic a month later for a checkup and tests. Receive a phone call 1 week after vaccination, and 2 months after vaccination, for health checks.

Keep a diary of their symptoms for 7 days after vaccination.

This is a Phase 3, randomized, double-blinded, placebo-controlled, multicenter trial to describe the safety, tolerability, and immunogenicity of bivalent RSVpreF in adults 60 years of age and older in Korea.

The study duration is approximately 2 months. 4 study visits are required and are comprised of 2 scheduled clinic visits and 2 scheduled telephone calls.

Approximately 360 study-eligible participants will be randomized to receive either the 120-µg dose of RSVpreF or placebo in a 2:1 ratio.

After screening and confirmation of eligibility, a prevaccination blood sample will be collected for immunogenicity assessments and a single dose of study intervention (RSVpreF or placebo) will be administered.

Participants will report daily reactogenicity data using an electronic device for 7-days or until resolution.

Participants will return approximately 1 month later for a follow-up blood draw for immunogenicity assessments and collection of safety information.

A telephone follow-up visit will be conducted approximately 1 week after vaccination to review reactogenicity and approximately 2 months after vaccination to collect safety information.

For all participants, adverse events (AEs) will be collected from informed consent through 1 month following study intervention administration. Serious adverse events (SAEs) newly diagnosed chronic medical conditions (NDCMCs), and adverse events of special interest (AESIs) will be collected from informed consent throughout study participation.

Inclusion Criteria:

1. Participants 60 years of age or older at Visit 1

   • Male participants able to father children must agree to use a highly effective method of contraception from the time of informed consent through at least 28 days after study intervention administration
   • Female participants must not be of childbearing potential

2. 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.

   Healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 6 weeks before enrollment, can be included.

3. Participants who are willing and able to comply with all scheduled visits, investigational plan, laboratory tests, frequent symptom assessment by mobile device application (e-diary), and other study procedures.

4. Participants who are ambulatory and live in the community, or in assisted-living or long-term care residential facilities that provide minimal assistance, such that the participant is primarily responsible for self-care and activities of daily living (ADL).

5. Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent document (ICD) and the protocol.

Exclusion Criteria:

1. A confirmed diagnosis of RSV infection ≤180 days before study intervention administration.

2. Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular (IM) injection.

3. Prior history of any subtype of Guillain-Barré syndrome (GBS) of any etiology.

4. History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis) to any component of the study intervention(s) or any related vaccine.

5. Serious chronic disorder, including metastatic malignancy, end-stage renal disease with or without dialysis, clinically unstable cardiac disease, or any other disorder that, in the investigator's opinion, excludes the participant from participating in the study.

6. Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.

7. Any 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.

8. Individuals who receive chronic systemic treatment with immunosuppressive therapy (other than systemic corticosteroids meeting the criteria noted below), including cytotoxic agents, immunosuppressive monoclonal antibodies, or radiotherapy, eg, for cancer or an autoimmune disease, from 60 days before study intervention administration or planned receipt throughout the study.

   • Receipt of systemic corticosteroids (≥20 mg/day of prednisone or equivalent) for

     ≥14 days from 28 days before study intervention.

   • Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

9. Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration.

10. Previous vaccination with any licensed or investigational RSV vaccine, or planned receipt throughout the study.

11. Previous administration with an investigational product (drug or vaccine) within 6 months prior to study intervention administration. Participation in other studies involving an investigational product (drug or vaccine) at any time during participation in this study.

12. Investigator site staff directly involved in the conduct of the study and their family members, site staff otherwise supervised by the investigator, and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members.

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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.

This study is double-blinded.

The participant, study coordinator, and all site staff will be blinded.

The majority of sponsor staff will be blinded to study intervention allocation. All laboratory testing personnel performing serological assays or diagnostic assays will remain blinded to the study intervention assigned/received throughout the study