A Study to Evaluate Safety, Reactogenicity and Immunogenicity of GSK Biologicals' RSV Investigational Vaccine Based on Viral Proteins Encoded by Chimpanzee-derived Adenovector (ChAd155-RSV) (GSK3389245A) in RSV-seropositive Infants

The purpose of this study is to evaluate the safety, reactogenicity and immunogenicity of the respiratory syncytial virus (RSV) candidate vaccine when first administered via intramuscular (IM) injection according to a 0, 1-month schedule to RSV-seropositive infants aged 12 to 23 months.

The RSV PED-002 study, designed to evaluate the safety, reactogenicity and immunogenicity of the RSV candidate vaccine when administered in 3 sequential doses to seropositive infants aged 12 to 23 months, will be conducted in an observer-blind manner in Epoch 1 and single-blinded in Epoch 2.

Eligibility Criteria:
Inclusion Criteria:

* Subjects' parent(s)/ Legally acceptable representative (LAR\[s\]) who, in the opinion of the investigator, can and will comply with the requirements of the protocol.
* Written informed consent obtained from the parent(s)/LAR(s) of the subject prior to performance of any study specific procedure.
* A male or female between, and including, 12 and 23 months at the time of the first vaccination.
* Healthy subjects as established by medical history and clinical examination before entering into the study.
* Seropositive for RSV as determined by IBL International kit.
* Born full-term (i.e. after a gestation period of 37 to less than 42 completed weeks) with a minimum birth weight of 2.5 kg. (Required for Spain)
* Subjects' parent(s)/LAR(s) need to have access to a consistent mean of telephone contact or computer.

Exclusion Criteria:

* Child in care.
* Use of any investigational or non-registered product other than the study vaccine during the period starting 30 days before the first dose of study vaccine (Day -29 to Day 0), or planned use during the study period.
* Any medical condition that in the judgment of the investigator would make IM injection unsafe.
* Chronic administration of immunosuppressants or other immune-modifying drugs during the period starting six months prior to the first vaccine. For corticosteroids, this will mean prednisone, or equivalent. Inhaled and topical steroids are allowed.
* Administration of long-acting immune-modifying drugs or planned administration at any time during the study period.
* Administration of immunoglobulins and/or any blood products during the period starting three months before the first dose of study vaccine or planned administration during the study period.
* Planned administration/administration of a vaccine not foreseen by the study protocol in the period starting 30 days before the first dose and ending 30 days after the last dose of vaccine administration, with the exception of scheduled routine pediatric vaccines which may be administered ≥ 14 days before a dose or ≥ 7 days after a dose.
* Acute or chronic, clinically significant pulmonary, cardiovascular, hepatic or renal functional abnormality, as determined by physical examination or laboratory screening tests.
* Serious chronic illness.
* Major congenital defects.
* History of any neurological disorders or seizures.
* History of or current autoimmune disease.
* History of recurrent wheezing.
* History of chronic cough.
* Previous hospitalization for respiratory illnesses.
* History of thrombocytopenia.
* History of anemia.
* Previous, current or planned administration of Synagis.
* Neurological complications following any prior vaccination.
* Born to a mother known or suspected to be HIV-positive.
* Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination.
* Family history of congenital or hereditary immunodeficiency.
* Previous vaccination with a recombinant simian or human adenoviral vaccine.
* History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccine.
* Hypersensitivity to latex.
* Current severe eczema.
* Acute disease and/or fever at the time of enrolment.

  * Fever is defined as temperature ≥ 37.5°C/99.5°F for oral, axillary or tympanic route, or ≥ 38.0°C/100.4°F for rectal route. The preferred route for recording temperature in this study will be axillary.
  * Clinically significant upper respiratory tract infection
  * Subjects with a minor illness without fever may, be enrolled at the discretion of the investigator.
* Any clinically significant Grade 1 or any ≥ Grade 2 hematological or biochemical laboratory abnormality detected at the last screening blood sampling.
* Any other conditions that the investigator judges may interfere with study procedures or findings.
* Any conditions that could constitute a risk for the subjects while participating to this study.
* Weight below the fifth percentile of the local weight-for-age curve.
* Concurrently participating in another clinical study, at any time during the study period, in which the subject has been or will be exposed to an investigational or a non-investigational vaccine/product.
* Planned move to a location that will prohibit participating in the trial until study end.