A Safety and Immune Study of 2 Types of GlaxoSmithKline's Varicella Vaccines Given as a 2-doses Course to Healthy Children 12-23 Months of Age.

Inclusion Criteria: * Subjects' parent(s)/ LAR(s) who, in the opinion of the investigator, can and will comply with the requirements of the protocol. * A male or female between, and including, 12 and 23 months of age (i.e. 12 months to a day before 24 months) at the time of the first study vaccination. * Written informed consent obtained from the parent(s)/LAR(s) of the subject prior to performing any study specific procedure. * Subjects in stable health as determined by investigator's clinical examination and assessment of subject's medical history. * Subjects must have had prior administration of a dose of measles, mumps and rubella (MMR) vaccine at least 30 days (Day -31 or earlier) prior to study vaccination at Day 0. Exclusion Criteria: * Child in care. * Use of any investigational or non-registered product other than the study vaccines during the period starting 30 days before the day of study vaccination (i.e., 30 days prior to Visit 1/Day 0) or planned use during the entire study period. * Concurrently participating in another clinical study, in which the child has been or will be exposed to an investigational or a non-investigational product. * Chronic administration (defined as 14 or more consecutive days) of immunosuppressants, or other immune-modifying drugs during the period starting 180 days prior to the first vaccine dose or any planned administration of immunosuppressive and immune-modifying drugs during the entire study. * For corticosteroids, this will mean prednisone ≥0.5 mg/kg/day or equivalent. * Inhaled and topical steroids are allowed. * Planned administration/ administration of a live viral vaccine not foreseen by the study protocol during the period starting 30 days prior to study vaccination at Visit 1/Day 0 until study end. Non study live viral vaccines can be administered at Visit 3 (Day 84) after completion of study procedures. * Planned administration/ administration of an inactivated vaccine not foreseen by the study protocol during the period starting 7 days prior to each vaccination (at Visit 1/Day 0 and Visit 2/Day 42) and ending 14 days after each vaccination. Outside of this period, non-study inactivated vaccines can be administered as per standard of care. * Administration of immunoglobulins and/or any blood products during the period starting 180 days prior to the first vaccine dose or planned administration from the date of first study vaccination through the entire study. * History of varicella or zoster. * Known exposure to varicella/zoster during the period starting within 30 days prior to first study vaccination. * Previous vaccination against varicella. * Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination (no laboratory testing required). * Subjects with blood dyscrasias, leukemia, and lymphomas of any type. * A family history of congenital or hereditary immunodeficiency * History of allergic disease or reactions likely to be exacerbated by any component of the vaccines, including hypersensitivity to neomycin or latex. * Major congenital defects or serious chronic illness. * Acute disease and/or fever at the time of enrolment. * Fever is defined as temperature ≥38. 0°C/100.4°F by any age appropriate route. * Subjects with a minor illness without fever may be enrolled at the discretion of the investigator. * Active untreated tuberculosis based on medical history. * Any other condition which, in the opinion of the investigator, prevents the child from participating in the study.