This clinical trial is being done to learn more about how well a vaccine (Arexvy®) for respiratory syncytial virus, also known as RSV, works in people with weakened immune systems. The main questions it aims to answer are:

• Does 1 or 2 doses of Arexvy work better in people with weakened immune systems?
• What medical problems do participants have after receiving Arexvy?

Participants with weakened immune systems will:

• Receive 3 study vaccines over the course of 1 year
• Keep a diary of symptoms for 7 days after each vaccine
• Have 3 in-person follow up study visits for checkups and tests over the course of 1.5 years
• Have 6 phone follow up study visits over the course of 1.5 years

People with weakened immune systems will be randomized to either:

1. Receive one dose of Arexvy followed by a placebo vaccine (sterile saline) 60 days later, or
2. Receive one dose of Arexvy followed by another dose of Arexvy 60 days later

Both groups will also receive another dose of Arexvy 1 year after the first dose.

A small group of people without weakened immune systems will also be enrolled in the study. This group will receive one dose of Arexvy.

Inclusion Criteria:

• Able to understand and provide informed consent

• Willing and able to comply with the requirements and restrictions in the protocol, including study visits and study-related procedures

• Medically stable in the opinion of the Investigator at the time of first study vaccination

• Life expectancy ≥ 365 days in the opinion of the Investigator at the time of first study vaccination

• Included in at least one of the groups below:

  1. Cellular therapy recipients (CTR):

     • Individuals ≥ 18 years of age at the time of first study vaccination
     • History of at least one of the following: i. Autologous stem cell transplant received more than 90 days prior to first study vaccination, ii. Allogeneic stem cell transplant received more than 90 days prior to first study vaccination, iii. Chimeric antigen receptor T cell (CAR-T) therapy received more than 90 days prior to first study vaccination

  2. Solid organ transplant recipients (SOTR):

     • Individuals ≥ 18 years of age at the time of first study vaccination
     • Received an ABO-compatible, single-organ type, solid organ transplant (lung, heart, kidney, liver) at least 90 days prior to first study vaccination, and are on at least 2 systemic immunosuppressive agents at time of first study vaccination

  3. Healthy comparator (HC):

     • Individuals ≥ 60 years of age at the time of first study vaccination or 50-59 years of age at the time of first study vaccination and at increased risk of severe RSV disease
     • No history of (a) or (b) above, and considered healthy or with chronic and stable medical conditions in the opinion of the Investigator, without immune compromise

• Participants of childbearing potential if practicing adequate contraception or abstinence from 1 month prior to first study vaccination and agree to continue adequate contraception or abstinence through at least 1 month after last study vaccination. All participants of childbearing potential must have a negative pregnancy test on the day of first study vaccination, prior to administration.

Exclusion Criteria:

• Known history of hypersensitivity to any vaccine or history of a life-threatening reaction to a vaccine

• Previous vaccination with any licensed or investigational RSV vaccine

• Acute or chronic clinically significant/unstable neurological disease (such as uncontrolled seizures, strokes, Guillain-Barré Syndrome (GBS)

• Vaccination with any inactivated, subunit, or split influenza vaccine or COVID-19 vaccine within 14 days prior to first study vaccination, or vaccination with any other licensed or investigational vaccine within 30 days prior to first study vaccination

• Receipt of investigational or approved monoclonal antibodies against RSV within 90 days prior to first study vaccination

• Moderate or severe acute illness/infection (in opinion of the Investigator) or febrile illness (temperature ≥ 38.0°C [≥ 100.4°F]) on the day of first study vaccination. A prospective participant should not be enrolled in the study until the condition has resolved or the febrile event has subsided.

• Any medical condition that in the opinion of the Investigator would make intramuscular injection unsafe

• Receipt of immunoglobulins or plasma products within 90 days prior to first study vaccination

• Receipt of B-cell depleting medications (e.g., Rituximab, ocrelizumab, ofatumumab, belimumab, epratuzumab, antithymocyte globulin) within 90 days prior to first study vaccination

• Currently pregnant or breastfeeding or planning to become pregnant, discontinue contraception, or breastfeed during the study period

• Any of the following:

  1. Cellular therapy recipients (CTR):

     • Graft-versus-host disease (GVHD) requiring systemic treatment with at least 0.5 mg/kg per day of prednisone or equivalent at time of first study vaccine

  2. Solid organ transplant recipients (SOTR):

     • History of any of the following within 90 days prior to first study vaccination: allograft rejection, post-transplant lymphoproliferative disease, treatment for either of these conditions

  3. Healthy comparator (HC):

     • Any confirmed/suspected immunosuppressive or immunodeficient condition resulting from disease or immunosuppressive or cytotoxic therapy, based on medical history

• Any other conditions which, in the opinion of the Investigator, may pose additional risks from participation in the study, may interfere with the individual's ability to comply with study requirements or may impact the quality or interpretation of the data obtained from the study