A Study on the Immune Response and Safety of Vaccine Against Respiratory Syncytial Virus (RSV) Given to Adults 18 to 49 Years of Age at Increased Risk for Respiratory Syncytial Virus Disease, Compared to Older Adults 60 Years of Age and Above

Inclusion criteria

• Participants and/or participant's parent(s)/ Legally acceptable representative (LAR) who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of the eDiary, attend study site visits, ability to access and utilize a phone or other electronic communications).
• Written or witnessed informed consent obtained from the participant/participant's parent(s)/LAR(s) (participant must be able to understand the informed consent) prior to performance of any study-specific procedure.

Written informed assent obtained from the participant (participant must be able to understand the informed assent) if he/she is less than the legal age prior to performance of any study-specific procedure.

Specific inclusion criteria for all participants in Cohort 1 and Cohort 3 (RSV-A-AIR Group) • A male or female participant 18-49 YOA at the time of the study intervention administration.

• Participants should be diagnosed with at least 1 of the following medical conditions if considered medically stable by the investigator:

• Chronic cardiopulmonary disease resulting in activity restricting symptoms or use of long term medication:

o Chronic obstructive pulmonary disease (COPD)

o Asthma

o Cystic fibrosis

o Other chronic respiratory diseases: lung fibrosis, restrictive lung disease, interstitial lung disease, emphysema or bronchiectasis

o Chronic heart failure:

o Pre-existing Coronary Artery Disease (CAD) (CAD not otherwise specified)

• Cardiac arrhythmia
• Diabetes mellitus: types 1 or 2 with active treatment for the past 6 months
• Other diseases at increased risk for RSV disease:

• Chronic kidney disease
• Chronic moderate to severe liver disease
• Neurologic or neuromuscular conditions
• Female participants of non-childbearing potential may be enrolled in the study. Non-childbearing potential is defined as premenarche, hysterectomy, bilateral oophorectomy, bilateral salpingectomy or post-menopause.
• Female participants of childbearing potential may be enrolled in the study, if the participant:

• has practiced adequate contraception from 1 month prior to study intervention administration, and
• has a negative pregnancy test on the day of study prior to intervention administration, and
• has agreed to continue adequate contraception for at least 1 month after completion of the study intervention administration.

Specific inclusion criteria for all participants in Cohort 2 (RSV-OA Group):

• A male or female participant >=60 YOA at the time of the study intervention administration.

Participants with chronic stable medical conditions with or without specific treatment, such as diabetes, hypertension or cardiac disease are allowed to participate in this study if considered medically stable by the investigator.

• Participants living in the general community or in an assisted-living facility that provides minimal assistance, such that the participant is primarily responsible for self-care and activities of daily living.

Exclusion criteria

Medical conditions

• Any confirmed or suspected immunosuppressive or immunodeficient condition resulting from disease (e.g., current malignancy, human immunodeficiency virus) 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).
• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study intervention.
• Unstable chronic illness.
• Any history of dementia or any medical condition that moderately or severely impairs cognition.
• Recurrent or uncontrolled neurological disorders or seizures. Participants with medically controlled active or chronic neurological diseases can be enrolled in the study as per investigator assessment, provided that their condition will allow them to comply with the requirements of the protocol (e.g., completion of the diary cards, attend study site visits). Study participants may decide to assign a caregiver to help them complete the study procedures.
• Significant underlying illness that in the opinion of the investigator would be expected to prevent completion of the study (e.g., life-threatening disease).
• Any medical condition that in the judgment of the investigator would make intramuscular injection unsafe.
• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the study.

Prior/Concomitant therapy

• Use of any investigational or non-registered product (drug, vaccine, or medical device) other than the study intervention during the period beginning 30 days before the dose of study intervention (Day -29 to Day 1), or planned use during the study period (up to Visit 3, Month 6).
• Planned or actual administration of a vaccine not foreseen by the study protocol in the period starting 30 days before and ending 30 days after the dose of 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.
• Previous vaccination with any RSV vaccine, including investigational RSV vaccines.
• Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or administration of long-acting immune-modifying treatments or planned administration at any time up to the End-of-study (EOS).

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

• For corticosteroids, this will mean prednisone >=20 mg/day, or equivalent. 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., Tumor Necrosis Factor (TNF)-inhibitors), monoclonal antibodies, antitumoral medication.

Prior/Concurrent clinical study experience

• 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 vaccine/product (drug or invasive medical device).

Other exclusions:

Other exclusions for all participants:

• 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.
• Bedridden participants.
• Planned move during the study period that will prohibit participating in the study until study end.
• Participation of any study personnel or their immediate dependents, family, or household members.

Other exclusions for Cohort 1 and Cohort 3:

• Pregnant or lactating female participant.
• Female planning to become pregnant or planning to discontinue contraceptive precautions within 1 month after study intervention administration.