I have a 16 months old son with CAH. He had a MMR shot when he was 12 months old. There was some side effects from it after about 10 days from the day he got the shot. He had a fever going up and down for a few days and crazy rushes......they appeared and gone so quick, like in 10 minutes. I increased his cortef a couple of times, but there was nothing serious. He still played a lot and ate a lot. I don't know how effective the shot is, but I believe it's worth it.