The purpose of this study is to learn about the safety and immunogenicity of a study vaccine (called RSVpreF) in several adult groups. Respiratory Syncytial Virus (RSV) is a common type of virus (germ) that can cause severe illness, where medical help is needed. RSV can lead to airway diseases in all ages. Vaccines help your body make antibodies which help fight against diseases. This is called an immune response. This study will measure how much antibody participants make after receiving RSVpreF (immunogenicity).

The study consists of 2 groups (Substudy A and Substudy B).

Substudy A is seeking approximately 675 participants who are:

• Between 18 and 60 years of age.
• Considered having a high likelihood of severe RSV disease due to certain long-term medical conditions. Such medical conditions do not include immunocompromising conditions.

Participants will need to come to the study clinic at least 2 times. At the first clinic visit, participants will receive 1 shot of RSVpreF or placebo in the arm by chance. A placebo looks like the study vaccine but contains no active ingredients. At each clinic visit, a blood sample will be taken. A third (final) visit can be either completed in clinic or via telephone contact. This study is about 6 months long for each participant.

Substudy B is seeking approximately 200 participants who are:

• At least 18 years of age. About half of the participants will be at least 60 years of age.
• Considered having a weakened immune system (immunocompromised). Participants will need to come to the study clinic at least 3 times. All participants will receive a shot of RSVpreF at the first study clinic visit. The second study clinic visit will be 1 month later. All participants will receive a second shot of the study vaccine at this second study clinic visit. Blood samples will be taken at the 3 study clinic visits. A fourth (final) visit can be either completed in clinic or via telephone contact. This study is about 7 months long for each participant.

This is a Phase 3 protocol that will assess the safety, tolerability, and immunogenicity of Pfizer's RSVpreF in adults at high risk of severe RSV disease. Each substudy design is detailed separately, and these substudies may be conducted in parallel, as required by the clinical plan, within the framework of this protocol.

Substudy A Design:

This is a Phase 3, multicenter, randomized, double-blinded, placebo-controlled study that will assess the safety, tolerability, and immunogenicity of Pfizer's RSVpreF in adults 18 to <60 years of age considered to be at high risk of RSV disease due to certain chronic medical conditions.

Approximately 675 participants ≥18 to <60 years of age considered at high risk of RSV disease due to certain chronic medical conditions, excluding immunocompromising conditions, will be randomized to receive a single 120-µg dose of RSVpreF or placebo in a 2:1 ratio. Enrollment will be monitored to help ensure distribution of vaccination across the age range. The duration of study participation for each participant will be 6 months, with 3 scheduled visits.

All participants will have blood drawn at baseline prior to vaccination and at 1 month after vaccination to assess immunogenicity. Immunogenicity elicited at 1 month after vaccination with RSVpreF in Substudy A will be bridged to the immunogenicity of participants 60 years of age and older in the C3671013 study, in which RSVpreF efficacy was demonstrated.

Local reaction and systemic event data will be collected in an e-diary for 7 days after study vaccination (Days 1 through 7, where Day 1 is the day of vaccination). Reported Grade 3 reactogenicity will be assessed by the study site to determine if an unscheduled visit is required.

For all participants, AEs will be collected from informed consent through 1 month following study intervention administration, and AESIs, NDCMCs and SAEs will be collected from informed consent throughout study participation. In addition, AEs occurring up to 48 hours after blood draws that are related to study procedures will be collected.

Substudy B Design:

This is a Phase 3, single-arm, open-label, multicenter study that will assess the safety, tolerability, and immunogenicity of Pfizer's RSVpreF in immunocompromised adults.

Substudy B included approximately 200 immunocompromised adults ≥18 years of age, that will receive 2 120-µg doses of RSVpreF with an interval of 1 month. Approximately 100 participants will be ≥60 years of age and approximately 100 participants will be ≥18 to 60 years of age. Enrollment will be monitored to help ensure distribution of vaccination across the age ranges and underlying immunocompromising conditions. The duration of study participation for each participant will be 7 months, with 4 scheduled visits. All participants will have blood drawn at baseline prior to vaccination and at 1 month after (each) vaccination to assess immunogenicity.

Local reaction and systemic event data will be collected in an e-diary for 7 days after study vaccination (Days 1 through 7, where Day 1 is the day of vaccination). Reported Grade 3 reactogenicity will be assessed by the study site to determine if an unscheduled visit is required.

For all participants, AEs will be collected from informed consent through 1 month following the last study intervention administration, and AESIs, NDCMCs and SAEs will be collected from informed consent throughout study participation. In addition, AEs occurring up to 48 hours after blood draws that are related to study procedures will be collected.

Substudy A Inclusion Criteria:

1. Capable of giving signed informed consent as described per protocol.

2. Participants ≥18 to <60 years of age at study enrollment.

3. Life expectancy ≥12 months (365 days) in the opinion of the investigator at enrollment.

4. Participants who are willing and able to comply with all scheduled visits, vaccination plan, lifestyle considerations, and other study procedures.

5. Participants who are considered at high risk of RSV disease by virtue of the following:

   • Adults with chronic pulmonary (including asthma), cardiovascular (excluding isolated hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus).

Chronic medical conditions for this substudy are defined as:

- Duration greater than 6 months.

• Stable disease not requiring a significant change in therapy in the previous 6 weeks or hospitalization for worsening disease within 12 weeks before receipt of study intervention.

• Requires regular medical follow-up or ongoing medication or hospitalization in the previous year.

  • Additional groups at high risk include:

• Residents of nursing homes and other long-term care facilities.

Substudy A Exclusion Criteria:

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

2. 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.

3. Participants who do not have adequate deltoid muscle mass to allow intramuscular vaccination, in the opinion of the investigator.

4. 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.

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

6. 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.

   Note: Specific criteria for participants with known stable infection with HIV can be found in protocol.

7. Individuals who receive chronic systemic treatment with immunosuppressive therapy, including cytotoxic agents, immunosuppressive monoclonal antibodies, systemic corticosteroids*, eg, for cancer or an autoimmune disease, or radiotherapy, from 60 days before study intervention administration or planned receipt throughout the study.

   *Applies to systemic corticosteroids administered for ≥14 days at a dose of ≥20 mg/day of prednisone or equivalent (eg, for cancer or an autoimmune disease). Systemic corticosteroids administered at a dose of <20 mg/day of prednisone or equivalent are permitted. Inhaled/nebulized, intra-articular, intrabursal, or topical (skin, eyes, or ears) corticosteroids are permitted.

8. Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration or planned receipt of these medications prior to the final blood draw.

   Note: Monoclonal antibodies with targeted mechanisms of action used in the management of chronic illnesses (eg, migraine headaches, osteoporosis) are permitted, provided they do not meet exclusion criterion 7.

9. Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.

10. Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-month follow-up visit.

11. 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.

Substudy B Inclusion Criteria

1. Capable of giving signed informed consent as described per protocol.

2. Participants ≥18 years of age at study enrollment.

3. Life expectancy ≥12 months (365 days) in the opinion of the investigator at enrollment.

4. Participants who are willing and able to comply with all scheduled visits, vaccination plan, lifestyle considerations, and other study procedures.

5. Participants who are immunocompromised by virtue of the following:

   •Having known advanced NSCLC with at least 1 of the following:

   • Has received initial or maintenance chemotherapy at least 2 weeks (14 days) before enrollment (or is treatment-naïve), and is not expected to receive chemotherapy within at least 2 weeks (14 days) after dose administration of initial vaccination or second vaccination; and/or
   • Is receiving checkpoint inhibitor treatment (PD-1/PD-L1 inhibitor, CTLA-4 inhibitor) and has undergone at least 1 treatment cycle prior to enrollment; or
   • Is receiving targeted drug therapy (EGFR, ALK, ROS1, BRAF, RET, MET, NTRK inhibitors) and has undergone at least 1 treatment cycle prior to enrollment.

   OR - Is currently undergoing maintenance hemodialysis treatment secondary to end-stage renal disease . OR - Is on active immunomodulator therapy (eg, TNFα inhibitor, tofacitinib, or MTX) for an autoimmune inflammatory disorder (eg, inflammatory arthritis, such as rheumatoid arthritis, psoriatic arthritis, and juvenile idiopathic arthritis, or inflammatory bowel disease, such as ulcerative colitis or Crohn's disease) at a stable*dose.

   *Stable dose is defined as receiving the same dose for at least 3 months (84 days) with no changes in the 28 days prior to enrollment. See protocol for details on stable dose for MTX.

   OR - Is receiving an SOT (kidney, liver, lung, or heart) at least 3 months (84 days) prior to enrollment (Visit 201) and with no acute rejection episodes within 2 months (60 days) prior to enrollment (Visit 201).

   Substudy B Exclusion Criteria:

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

2.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.

3.Participants with a history of transplant rejection, or PTLD, or participants who have had treatment for these conditions within 3 months (84 days) prior to study enrollment.

4.Participants who do not have adequate deltoid muscle mass to allow intramuscular vaccination, in the opinion of the investigator.

5.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.

Note: Specific criteria for participants with known stable infection with HIV can be found in protocol.

6.Receipt of investigational or approved monoclonal antibodies against RSV within 6 months before study intervention administration.

7.Receipt of blood/plasma products or immunoglobulin (IVIG, SCIG) within 60 days before study intervention administration or planned receipt of these medications prior to the final blood draw.

Note: Please see the inclusion criteria in the protocol regarding criteria for targeted immunoglobulin therapies for underlying medical conditions.

Note: Monoclonal antibodies with targeted mechanisms of action used in the management of chronic illnesses (eg, migraine headaches, osteoporosis) are permitted.

8.Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.

9.Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-month follow-up visit.

10.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.

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.

Substudy A is double blind therefore all parties (participant, site staff and sponsor) will be blinded to study intervention.

Substudy B is open-label therefore no blinding requirements are in place since all participants will receive RSVpreF.