RecruitingPhase 3Biological interventionA Study on the Safety of an Investigational Chickenpox Vaccine, When Given to Healthy Children, 12 to 15 Months of AgeThe purpose of this study is to assess how well-tolerated GSK's investigational varicella vaccine (VNS Vaccine) is, in comparison to an already approved varicella vaccine (VV) known as Varivax. The study will be conducted on healthy children aged 12 to 15 months, and who have neither contracted varicella nor received a varicella vaccination.