The study is powered to evaluate equivalence between the two strategies under the assumption that, in the control arm, the primary end-point would be observed in a proportion of subjects < 7% and that the same proportion in the STI arm would not exceed 10% with a maximum allowed 95%CI of 12%. According to the model published in Controlled Clinical Trials 1982; 3: 345-353,320 patients will be needed for alfa = 5% and 1-beta = 80%.