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Read myth 2. Yes vaccines can cause adverse affects but they are rare and, in many cases, predictable. Sometimes in order to be predictable they would first require genetic screening (which is expensive) and a lot of testing. sing the information on the vaccine from the CDC website here you can look at the risk assessment to find out how rare it was (though I know this is no consolidation) for his exact condition. Another question comes with severity (as DTaP vaccines are not linked to any long term severe conditions) is whether it was coincidence that something happened after the vaccine or if it was actually cause by the vaccine (as so few people get moderate let alone serious)

So never did I say they are harmless, what I said was (in myth 2) that vaccines do have the potential to cause adverse effects (vaccine dependent) but everything in medicine has the potential to cause adverse affects. Considering a majority of people that receive the vaccine not only survive but survive with minimal no no adversities (even including minor ones) with exception of a sore arm shows that the vaccine has more of a potential to help then to harm. I am sorry about your son, but he isn't enough to classify them as dangerous, especially with the vague story that doesn't even bring in symptoms to confirm with other cases.