The study consists of 3 parts, part A, encompassing Visit 1 to Visit 8, covering 3 priming immunizations at a dose of 15μg at days 0, 28 and 56 and the immediate observation period extending to day 140; the prolonged observation period: part B, encompassing Visit 9 and Visit 10 at days 273 and 365; and part C consisting of 7 visits (V11 to V17). Study participants having received 3 priming vaccinations and having completed part B will receive in part C one boost immunization at a dose of 75μg, which will be applied one year after the 3rd immunization (day 420). Probands will proceed directly from part A to part B and to part C. Continuation of parts B and C will be considered based on the part A results, primarily the immunological results. The following scenarios apply (provided that there is no safety issue). None of the two treatment groups exhibits a vaccine-specific antibody response over the placebo group at Visit 8 - this will lead to termination of the trial. One of the two groups fails to exhibit a vaccine-specific antibody response over the placebo group at Visit 8 - this will lead to its discontinuation; the other treatment group and the placebo group will be continued.