 Statistical considerations : The number of patients necessary to detect the expected difference after 3 years of study 50 of recurrence in vaccinated group versus 70 in placebo group, with a statistical power of 80 and a bilateral test at 5, is 103 children per group. To assume the lost of statistical power in the intention to treat analysis ITT resulting from the number of cases where vaccine protocol has not been completed, 125 children per group will be included in the study. In total 250 children will be included in the study. 