The paper by Teixeria1 makes the case that a number of untoward effects have been linked to immunizations. Adler2 challenges that view. Each author wrote a scholarly piece, well conceived and with conclusions well supported by up-to-date literature. However, each arrived at opposite conclusions from the published literature he chose to consider. This indicates once again that science is not the objective process we would like it to be, but a process in which, as in many human endeavours, a priori bias is critical for interpreting available data. This variability in data interpretation can also be found within institutions or even countries. For example, while immunisation against tuberculosis (BCG) is mandatory in France, it is generally discouraged in the USA where it is considered only in very rare specific situations.