• Seroconversion rates for varicella antibodies [ Time Frame: post-dose 1 and post-dose 2 ]
• GMTs for varicella antibodies [ Time Frame: post-dose 2 ]
• Solicited symptoms after each vaccination (pain, redness, swelling: Day 0-3; fever; rash: Day 0-42) [ Time Frame: Day 0-3, Day 0-42 ]
• Unsolicited AEs after each vaccination [ Time Frame: Day 0-42 ]
• SAEs [ Time Frame: from dose 1 up to study end ]

The aim of this study is to evaluate a modified formulation of GSK Biologicals' live attenuated varicella vaccine. In vivo pre-clinical data show this change has no negative impact on vaccine safety. This present study is undertaken to rule out any negative impact on the immunogenicity and safety of GSK Biologicals' live attenuated varicella virus vaccine.

Inclusion Criteria:

• Subjects who the investigator believes that their parents/guardians can and will comply with the requirements of the protocol (e.g. completion of the diary cards, return for follow-up visits) should be enrolled in the study.
• A male or female between, and including, 11 and 21 months of age at the time of the first vaccination.
• Written informed consent obtained from the parent or guardian of the subject.
• Healthy subjects as established by medical history and clinical examination before entering into the study.

Exclusion Criteria:

• Use of any investigational or non-registered product (drug or vaccine) other than the study vaccine(s) within 30 days preceding the first dose of study vaccine, or planned use during the study period.
• Chronic administration of immunosuppressants or other immune-modifying drugs within six months prior to the first vaccine dose.
• Planned administration/ administration of a vaccine not foreseen by the study protocol within 30 days prior to the first study vaccine dose until 42 days after second study vaccine dose with the exception of oral polio vaccine (OPV) which can be given at any time and routine inactivated vaccines which can be administered up to eight days before each study vaccine dose.
• Previous vaccination against varicella.
• Known history of clinical varicella.
• Known exposure to varicella within 30 days prior to study start.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination.
• A family history of congenital or hereditary immunodeficiency.
• History of allergic disease or reactions likely to be exacerbated by any component of the vaccine(s), including systemic hypersensitivity to neomycin.
• Major congenital defects or serious chronic illness.
• Acute disease at the time of enrolment. All vaccines can be administered to persons with a minor illness.
• Axillary temperature ≥ 37.5°C / Rectal temperature ≥ 38°C.
• Residence in the same household as a high risk person e.g.: new-born infants (0-4 weeks of age), pregnant women who have a negative history of chickenpox, persons with known immunodeficiency