When he had his first, although in hindsight already autistic, he was really ill with awful D&V for a fortnight and his diet went right downhill afterwards. He had weaned perfectly, ate all sorts, but after the jab couldn't even keep water down let alone the dioralyte the emergency doctor recommended. I got him through it on BM but it was a struggle.
I have been under a lot of pressure to get this done by the HV at every single TAC meeting.
Have looked at options such as single jabs and having a blood test done to determine immune status but neither seem suitable. I am convinced that he is immune anyway because he had the so-called "measles-like illness" 
a month after MMR, swine flu, winter flu, chicken pox and came through them fine, no problems. Plus he still has BM (when he can catch me) which might count for something.
But we have have problems and flags raised wrt our family. I don't want to say any more but people here know that it isn't uncommon. So I'm getting it done because I don't want anything else to go on record.
I hope that because he is bigger now, in age 5-6 clothes and very strong in terms of fighting things that all will be well It pisses me right off though because there is a good chance that there is no need for it, and all the worry, at all.