Ways of knowing: Observation, experience, learning and reasoning. Please observe these parental comments: State of health of unvaccinated children - survey results Given these types of anecdotes, how would you assess their validity. I suppose, if you were the authority for vaccine efficacy and safety, you'd convene in-depth studies. For some reason, all of the studies that get done (at least the ones which I've read) seem to admit that there is room for concern but in the end recommend that vaccination continue as before. It's like, there may be people on the tracks ahead but we'll keep the train running full speed anyway!