Vaccination is unquestionably a very important tool for disease control. However, vaccines can cause adverse events that may damage the public trust. In recent experience in Brazil, the implementation of mass campaigns with MMR (measles, mumps, rubella) vaccine was associated with outbreaks of vaccine adverse events. The decision-making process regarding the continuation of the use of this vaccine proceeded in the context of uncertainties, fears and contradictions between information from the scientific literature and data generated by a post-licensure monitoring. This experience showed how important is to have a critical view of previous recommendations and to get local data addressing questions to guide the decision making process.