I agree that it is a really hard one too. I am not sure that we will ever have a conclusive answer because research on humans is so difficult and fraught with ethical dilemas.

I do, however, have a friend who has a younger brother who had a serious reaction to the measles jam (granted 30 years ago). He was a happy normal child the day before his jab and hours hours was convulsing and unfortunately has a mental age of about 3.

But even after knowing this I still think that the benefits outweigh the possible risks. If they didn't we wouldn't get into a car and drive on the roads.