[ceptimus]

The reason why the post hoc ergo propter hoc fallacy is relevant to the vaccines/autism subject is that vaccinations are routinely given to children at about the same age when autism traits begin to become apparent; also even those people who believe that vaccinations cause autism admit that this only happens to a small minority of vaccinated children.

Now, imagine that no child was ever diagnosed with autism in the hours or days following a vaccination - that would be amazing! It would show that the vaccine, as well as protecting against, say, measles as intended was also giving temporary immunity against developing autism!

Let's say we want to scientifically test whether vaccinations cause autism symptoms in three-year-old children during the week following vaccination. First we would need to establish a baseline of how many unvaccinated three-year-olds showed the first signs of autism in a given week: for the sake of argument we can say this happens to 1 child out of every 100,000. Now for every million vaccinations given to three-year-olds we would expect about ten of those children to show autism signs in the week following vaccination - even if when the vaccine has no causal effect on autism whatsoever.

This is exactly what scientists have already done.