No vaccine is 100% effective. However, with the except of people that have large-scale immune diseases, there is no good way to know if, for a particular vaccine, you will be in the 3% not immunized. So, firstly, you get the vaccine to get a 97% chance of being immunized. That's the first ****ing point.
The second ***ing point is herd immunity. When you pool together the non-immunizable and the people where vaccines fail, that's around 94-95% of the population. That mean if one of those kids goes to Africa, or meets some infected African kid here in the USA, they won't spread measles here to the other 5-6%. Even if one of those 5-6% gets the measles, the measles won't spread beyond them, because just about everyone they contact will be immune.