Randomisation strategy. Study subjects were randomised before each of the three vaccinations (represented by arrows): included subjects were randomised into three equally sized vaccine groups for the Year 1 vaccination, then in two equally sized vaccine groups in each of the subsequent years. Randomisation for vaccination in Years 2 and 3 was stratified for the vaccine received in previous years to ensure that a comparable number of subjects were included in each vaccine group. An additional Year 3 immunogenicity randomisation list was generated to select randomly a subset of approximately 30 subjects per vaccine stratum for blood sampling. Randomisation was also stratified for centre (not illustrated here). N = number of subjects randomised to each group (discontinuations are not represented).