The perception of the risk of a vaccine and its effectiveness has always been the subject of study. The human mind, in fact, tends to underestimate the effectiveness of a vaccine and overestimate its risk. But where does this problem come from and how does the human being think when he is talking about a vaccine? In this work, I will present some experimental data on the perception of the vaccine to deepen the studies on the perception of risk. The aim will be to highlight what are the strategies of thought adopted by the human beings when they must reason on these issues.