This is a Phase II /III, multi-center, randomized, double-blind, placebo controlled trial. Approximately 500 healthy volunteers, male or female, aged 18 years or older, will be randomized to one of the following two groups:

Group A: Vaccination twice at 2-week intervals (n = 250) Group B: Vaccination twice at 4-week intervals (n = 250)

Fifty subjects in each group will receive placebos.