Respiratory Syncytial Virus (RSV) Investigational Vaccine in Infants Aged 6 and 7 Months Likely to be Unexposed to RSV

The purpose of this study is to provide critical information on the safety, reactogenicity and immunogenicity profile of the investigational recombinant chimpanzee adenovirus Type 155-vectored RSV (ChAd155-RSV) vaccine in infants likely to be unexposed to RSV and will assess a single lower dose and a higher two dose regimen, before moving to future studies. This study will also assess if there is a risk of 'vaccine-induced enhanced RSV disease' after vaccination of these infants with the ChAd155-RSV vaccine.


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 performing any study specific procedure.
* A male or female between and including 6 and 7 months of age (from the day the infant becomes 6 months of age until the day before the infant achieves 8 months of age) at the time of the first vaccination.
* Healthy subjects as established by medical history and clinical examination before entering into the study.
* Born full-term with a minimum birth weight of 2.5 kilograms (kg).
* 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 1), or planned use during the study period.
* 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 ≥ 0.5 milligrams (mg)/kg/day (for pediatric subjects), or equivalent. 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. Scheduled routine pediatric vaccines may be administered ≥ 7 days before a dose of study vaccine or ≥ 7 days following a dose of study vaccine, with the exception of live viral 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.
* A history of, or on-going confirmed RSV disease or highly compatible clinical picture.
* Serious chronic illness.
* Major congenital defects.
* History of any neurological disorders or seizures.
* History of or current autoimmune disease.
* History of recurrent wheezing in the subject's lifetime.
* History of chronic cough.
* Previous hospitalization for lower respiratory illnesses.
* Previous, current or planned administration of Synagis (palivizumab).
* Neurological complications following any prior vaccination.
* Born to a mother known or suspected to be Human Immunodeficiency Virus (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 to any component of the vaccines (investigational or control) or placebo used in this study or any contraindication to them.
* Hypersensitivity to latex.
* Current severe eczema.
* Acute disease and/or fever at the time of enrolment (Visit 1).
* Any clinically significant Grade 1 or any ≥ Grade 2 hematological or biochemical laboratory abnormality detected at the last screening blood sampling.
* Any medical condition that in the judgment of the investigator would make intramuscular (IM) injection unsafe.
* Any other conditions that the investigator judges may interfere with study procedures, 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 according to the World Health Organization (WHO) weight- for- age tables. Participating in another clinical study, at any time during the study period, in which the subject or mother (if breastfeeding) 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.
* For Thailand only, subjects who have received Synflorix prior to enrolment.