Efficacy Study: This randomized, double-blinded, placebo-controlled Phase 3 study is designed to assess the safety, immunogenicity, and efficacy of a single dose of RSVpreF in the prevention of LRTI-RSV in adults:

• At a dose of 120µg.
• In adults 60 years of age and older.
• The duration of the study for each participant will be up to approximately 24 months.
• The study will be conducted in the United States, Canada, Netherlands, Finland, Argentina, Japan and South Africa.

Substudy A: This study is an extension of the efficacy study and was designed to evaluate the safety and immunogenicity of a second dose of RSVpreF when administered after a dosing interval of approximately 2 years:

• At a dose of 120µg (as studied in the Phase 3 Efficacy Study)
• Blood samples will be collected for antibody testing.
• The duration of the study for each participant will be up to approximately 18 months.
• The study will be conducted in the United States and Argentina.

Substudy B: This study was designed to evaluate the safety and immunogenicity of a second dose of RSVpreF when administered after a dosing interval of approximately 1 year:

• At a dose of 120µg (as studied in the Phase 3 Efficacy Study)
• Blood samples will be collected for antibody testing.
• The duration of the study for each participant will be up to approximately 18 months.
• The study will be conducted in Argentina.

Substudy C: This study was designed to evaluate the safety and immunogenicity of a second dose of RSVpreF when administered after a dosing interval of either 3 or 4 years:

• At a dose of 120µg (as studied in the Phase 3 Efficacy Study)
• Participants will receive either placebo or a second dose of RSVpreF approximately 3 or 4 years after receiving the initial dose of RSVpreF in the main efficacy study.
• Blood samples will be collected for antibody testing.
• The duration of the study for each participant will be up to approximately 24 months.
• The study will be conducted in the United States and Canada.

Efficacy Study

Inclusion Criteria:

• Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, frequent symptom assessment by mobile device application, and other study procedures, including collection of nasal swabs by themselves and by study staff when indicated.
• 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.

Note: 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. Specific criteria for participants with known stable infection with HIV, HCV, or HBV can be found in the protocol.

• Adults 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.

• Capable of giving signed informed consent as described in the protocol, which includes compliance with the requirements and restrictions listed in the ICD and in this protocol.

• Male or female participants ≥60 years of age.

  • 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.

Exclusion Criteria:

• Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.
• 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.
• 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.
• Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.
• 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.
• Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-months follow-up visit (Visit 3).

Note: This criterion does not apply to participants who are participating in a follow-up period for another study involving a study intervention that is an investigational drug or vaccine, if receipt of the last dose was at least 6 months prior to consenting for this study and there is no further dosing anticipated from the previous study during the participant's participation in this study - Individuals who receive chronic systemic treatment with immunosuppressive therapy, including cytotoxic agents, monoclonal antibodies, systemic corticosteroids, or radiotherapy, eg, for cancer or an autoimmune disease, from 60 days before study intervention administration or planned receipt throughout the study. If systemic corticosteroids have been administered short term (<14 days) for treatment of an acute illness, participants should not be enrolled in the study until corticosteroid therapy has been discontinued for at least 28 days before study intervention administration. Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

Note: Participants with COPD or asthma can be enrolled if chronic corticosteroids do not exceed a dose equivalent to 10 mg/day of prednisone.

• Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration.
• Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.
• Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.

Substudy A

Inclusion Criteria:

• Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, frequent symptom assessment by mobile device application, and other study procedures, including collection of nasal swabs by themselves and by study staff when indicated.
• 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.

Note: 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. Specific criteria for participants with known stable infection with HIV, HCV, or HBV can be found in the protocol.

• Adults 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.

• Capable of giving signed informed consent as described in the protocol, which includes compliance with the requirements and restrictions listed in the ICD and in this protocol.

• Male or female participants ≥60 years of age.

  • 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.

• Participants who received RSVpreF in the efficacy study.

• Participants who have a Visit 2 serology sample available for testing from the efficacy study, completed the end-of-Season 2 visit (Visit 5), and did not meet exclusion criteria throughout the efficacy study duration.

Exclusion Criteria:

• Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.
• 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.
• 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.
• Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.
• 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.
• Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-months follow-up visit (Visit 103).

Note: This criterion does not apply to participants who are participating in a follow-up period for another study involving a study intervention that is an investigational drug or vaccine, if receipt of the last dose was at least 6 months prior to consenting for this study and there is no further dosing anticipated from the previous study during the participant's participation in this study

- Individuals who receive chronic systemic treatment with immunosuppressive therapy, including cytotoxic agents, monoclonal antibodies, systemic corticosteroids, or radiotherapy, eg, for cancer or an autoimmune disease, from 60 days before study intervention administration or planned receipt throughout the study. If systemic corticosteroids have been administered short term (<14 days) for treatment of an acute illness, participants should not be enrolled in the study until corticosteroid therapy has been discontinued for at least 28 days before study intervention administration. Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

Note: Participants with COPD or asthma can be enrolled if chronic corticosteroids do not exceed a dose equivalent to 10 mg/day of prednisone.

• Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration.
• Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.

Note: This criterion does not include the receipt of RSVpreF in the efficacy study. Per inclusion criterion #6, receipt of RSVpreF in the efficacy study is required to participate in Substudy A.

• Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.
• Participant was confirmed by the sponsor to have previously received the study intervention more than once.

Substudy B

Inclusion Criteria:

• Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, frequent symptom assessment by mobile device application, and other study procedures, including collection of nasal swabs by themselves and by study staff when indicated.
• 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.

Note: 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. Specific criteria for participants with known stable infection with HIV, HCV, or HBV can be found in the protocol.

• Adults 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.

• Capable of giving signed informed consent as described in the protocol, which includes compliance with the requirements and restrictions listed in the ICD and in this protocol.

• Male or female participants ≥60 years of age.

  • 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. 6. Participants who received RSVpreF in the efficacy study.

• Participants who received RSVpreF in the efficacy study.

• Participants who have a Visit 2 serology sample available for testing from the efficacy study and did not meet exclusion criteria through Visit 4 of the efficacy study.

Exclusion Criteria:

• Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.
• 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.
• 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.
• Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.
• 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.
• Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-months follow-up visit (Visit 203).

Note: This criterion does not apply to participants who are participating in a follow-up period for another study involving a study intervention that is an investigational drug or vaccine, if receipt of the last dose was at least 6 months prior to consenting for this study and there is no further dosing anticipated from the previous study during the participant's participation in this study - Individuals who receive chronic systemic treatment with immunosuppressive therapy, including cytotoxic agents, monoclonal antibodies, systemic corticosteroids, or radiotherapy, eg, for cancer or an autoimmune disease, from 60 days before study intervention administration or planned receipt throughout the study. If systemic corticosteroids have been administered short term (<14 days) for treatment of an acute illness, participants should not be enrolled in the study until corticosteroid therapy has been discontinued for at least 28 days before study intervention administration. Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

Note: Participants with COPD or asthma can be enrolled if chronic corticosteroids do not exceed a dose equivalent to 10 mg/day of prednisone.

• Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration.
• Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.

Note: This criterion does not include the receipt of RSVpreF in the efficacy study. Per inclusion criterion #6, receipt of RSVpreF in the efficacy study is required to participate in Substudy B.

• Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.
• Participant was confirmed by the sponsor to have previously received the study intervention more than once.
• Participants who completed Vaccination 1 from the efficacy study less than 9 months or greater than 15 months prior to revaccination.

Substudy C

Inclusion Criteria:

• Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, frequent symptom assessment by mobile device application, and other study procedures, when indicated.
• 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.

Note: 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. Specific criteria for participants with known stable infection with HIV, HCV, or HBV can be found in the protocol.

• Adults 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.
• Capable of giving signed informed consent as described in the protocol, which includes compliance with the requirements and restrictions listed in the ICD and in this protocol.
• Male or female participants ≥60 years of age.
• Participants who received RSVpreF in the efficacy study.
• Participants who have a Visit 2 serology sample available for testing from the efficacy study, completed the end-of-Season 2 visit (Visit 5), and did not meet exclusion criteria throughout the efficacy study duration.

Exclusion Criteria:

• Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.
• 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.
• 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.
• Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination.
• 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.
• Participation in other studies involving an investigational product within 28 days prior to consent and/or through and including the 6-months follow-up visit (Visit 303 or Visit 306).

Note: This criterion does not apply to participants who are participating in a follow-up period for another study involving a study intervention that is an investigational drug or vaccine, if receipt of the last dose was at least 6 months prior to consenting for this study and there is no further dosing anticipated from the previous study during the participant's participation in this study

- Individuals who receive chronic systemic treatment with immunosuppressive therapy, including cytotoxic agents, monoclonal antibodies, systemic corticosteroids, or radiotherapy, eg, for cancer or an autoimmune disease, from 60 days before study intervention administration or planned receipt throughout the study. If systemic corticosteroids have been administered short term (<14 days) for treatment of an acute illness, participants should not be enrolled in the study until corticosteroid therapy has been discontinued for at least 28 days before study intervention administration. Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.

Note: Participants with COPD or asthma can be enrolled if chronic corticosteroids do not exceed a dose equivalent to 10 mg/day of prednisone.

• Receipt of blood/plasma products or immunoglobulin within 60 days before study intervention administration.
• Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation.

Note: This criterion does not include the receipt of RSVpreF in the efficacy study. Per inclusion criterion #6, receipt of RSVpreF in the efficacy study is required to participate in Substudy C.

• Investigator site staff or Pfizer employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members.
• Participant was confirmed by the sponsor to have previously received the study intervention more than once.
• Prior history of any subtype of Guillain-Barré syndrome of any etiology.
• Current or prior participation in Substudy A or Substudy B.
• Participants who completed Vaccination 1 from the efficacy study less than 32 months or greater than 40 months prior to revaccination.

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 is a double-blinded, placebo-controlled study.