The problem with the vaccine is that the effectiveness wears off after a while, but not for every kid at the same rate...and initially when the vaccine first came out there were more problems with it wearing off sooner (about 10 years ago when my kid got his dose). This is the same for the MMR, also. That's why you hear about college kids who live in dorms having outbreaks and needing to get re-vaccinated all the time...international students who have never been vaccinated, or kids who travel to other countries spread around the bugs.