There's another point that I can comment on from an anecdotal basis. My son, who is autistic, was very clearly autistic before his MMR vaccination - in fact, the first thing I ever noticed about him, within minutes of his birth, was that his responses and interactions weren't normal for a newborn baby. (It wasn't drug related because my wife didn't have any.) His MMR vaccination was an extremely traumatic experience, because we couldn't communicate with him effectively enough to reassure him that having a needle stuck in his arm wasn't going to cause him any long term harm, and his behaviour became a lot worse - meaning, effectively, a lot more autistic - immediately afterwards. If I hadn't already known that something wasunusual about his development, I could easily have made the false connection.
So, if another small child, already autistic, but not obviously so, goes for a vaccination, finds the experience traumatic because the impairment of his receptive communication makes him unable to receive the non-verbal signals of reassurance his parents are sending him - indeed, has never been able to receive these signals, leading to the high level of anxiety that's a classic feature of autism - and retreats, after the experience, to the rituals and repetitions that comfort him, and that his parents have never really seen as unusual before. But now he's doing it all a lot more, and it's too obvious to miss. And then they say, "But he wasn't doing anything like this before the vaccination." He was, but it didn't reach the significance threshold, so they didn't realise he was autistic.
Personally, I suspect that's the reason for the association. But it's only my opinion, and it's based on anecdotal evidence, so feel free to take it with a pinch of salt.
Dave
