causal relationship with one or more vaccines that may have beenadministered.Most of the solid evidence regarding the association of spe-cific adverse events with a particular vaccine comes from carefullydesigned epidemiological studies assessing absolute or relativerisks [5]. However, when investigating an individual case of adverseevent following immunization (AEFI), the probability, derived fromthese studies, that a causal association exists between a vaccine andan adverse event is not sufficient to draw definite conclusions, sinceother factors may intervene in the cause–effect relationship.