This 107066 study is a participating study to the broader NCT00502593 study, which included 2 other studies, the 108498 and 108500 studies. The NCT00502593 study had a staggered design (subjects from each group being enrolled sequentially into 2 age strata, '6-9 years' 1st, and '3-5 years' next) and was run in 3 phases, phases A (Study 107066), B (Study 108498) and C (Study 108500). In each phase, the decision to vaccinate subjects aged 3-5 years and to administer a 2nd injection to subjects aged 6-9 years was made based on safety data collected on Days 0-6 after the 1st injection for the subjects aged 6-9 years. Decision to start Phase B was made on complete safety data including those collected on Day 28 of Phase A (7 days after the 2nd injection) for each age group. A similar approach was to be used to start Phase C. As safety data in Phase A did not raise any safety concerns, Phases B and C were run in parallel.