The purpose of this study is to assess the consistency of immune response to three different lots of GSK's investigational varicella vaccine (VNS Vaccine), and to compare the safety and immune response of VNS vaccine to an already approved varicella vaccine (VV) known as Varivax. The study will be conducted in healthy children aged 12 to 15 months, who have neither contracted varicella nor received a varicella vaccination.