iirc NAS (or one of the autistic societies) believe that 7% of children with autism are autistic as a result of the mmr. (Am sure pag or jimjams or someone else will correct me if that's wrong.) Now that is quite a small risk by comparison, but there are lots of people who believe that there are certain children who are more scseptable (sp?) to autism due to certain conditions, ie bowel conditions and other auto-immune conditions, and that those children would therefore be more likely to regress as a result of vaccinations.
I don't think that calling people histerical is necessarily helpful. After all, there are people whose children have regressed following mmr - is it really a good idea to suggest that those people are histerical? I think not.
Op - ultimately only you can decide what you want to do. but I think that it's a very logical assumption to make that if a child has issues with immunity, then overloading that child's already compromised immune system with viruses isn't necessarily a wise move.
If you do a search on mn you will find many threads relating to vaccination, and many links posted by people like jimjams.
Just for the record, I remember my very first posting on mn about vaccinations, during which I said that people who didn't vaccinate were irresponsible because it was all histeria I was summarily jumped on by someone, and after that I did a lot more reading of the threads before posting and I now believe that it's really not as black and white as all that.
Vaccinations are safe for the majority of children. But if you have a child who fits into the auto immune group then there's nothing histerical about being concerned.