A Study to Evaluate the Safety, Tolerability and Immunogenicity of an Investigational RSV Vaccine Candidate (Ad26.RSV.preF) in Adults 18 to 50 Years of Age, and RSV-seropositive Toddlers 12 to 24 Months of Age

The study, designed to assess the safety and tolerability of two doses given one month apart of Ad26.RSV.preF, an investigational RSV vaccine candidate based on a Ad26 vector expressing the RSV F protein, will be conducted in a double blinded manner. The study will be divided in two sequential cohorts: cohort 0 (18-50 year-old adults) and cohort 1 (12-24 month-old RSV seropositive toddlers). The vaccine safety will be monitored by reporting solicited and unsolicited adverse events (AEs) and all serious adverse events (SAEs). The data will be reviewed by an independent data monitoring committee (IDMC) to assess safety data and to ensure the continuing safety of the participants enrolled in this study. The safety will be monitored throughout the study.

Inclusion criteria

Adults Participants:

• Participant must be in good health, without significant medical illness, on the basis of physical examination, medical history, and vital signs measurement
• Participant must be healthy on the basis of clinical laboratory tests performed at screening. If the results of the laboratory screening tests are outside the local laboratory normal reference ranges and additionally within the limits of toxicity Grade 1 according to the United stated (US) Food and Drug Administration (FDA) toxicity tables, the participant may be included only if the investigator judges the abnormalities or deviations from normal to be not clinically significant and appropriate and reasonable for the population under study. This determination must be recorded in the participant's source documents and initialed by the investigator
• All women of childbearing potential must have a negative highly sensitive serum beta-human chorionic gonadotropin (Beta-hCG) pregnancy test at screening and a negative urine Beta-hCG pregnancy test immediately prior to each study vaccine administration

Pediatric Participants:

• Participant is the product of a normal term pregnancy greater than and equal to (>=) 37 weeks, with a minimum birth weight of 2.5 kilogram (kg)
• Participants must be in good health without any significant medical illness on the basis of physical examination, medical history, and vital signs performed at screening

Exclusion criteria

Adults Participants:

• Participant has acute illness (this does not include minor illnesses such as diarrhea) or temperature >=38.0 ºC (degree celsius)/100.4 °F (fahrenheit) within 24 hours prior to the first dose of study vaccine
• Participant has a history of an underlying clinically significant acute or chronic medical condition or physical examination findings for which, in the opinion of the investigator, participation would not be in the best interest of the participant (example, compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments
• Participant has history of malignancy within 5 years before screening (exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or malignancy, which is considered cured with minimal risk of recurrence)

Pediatric Participants:

• Participant's weight is below 10th percentile according to World Health Organization (WHO) pediatric growth and weight charts
• Participant has any clinically significant acute or chronic medical condition that, in the opinion of the investigator, would preclude participation: example, history of seizure disorders, bleeding/clotting disorder, autoimmune disease, active malignancy, systemic infections, congenital heart disease, history of any pulmonary condition requiring medication, atopy, reactive airway disease, medically-confirmed wheezing, bronchoconstriction or treatment with a beta2 agonist, cystic fibrosis, bronchopulmonary dysplasia, chronic pulmonary disease, medically-confirmed apnea, hospitalization for respiratory illness, or mechanical ventilation for respiratory illness