My daughter (no CF) was vaccinated but still caught it - however it was a very mild case. Her best friend (who didn't get the vaccine) on the other hand missed a few weeks of school and was quite ill. She's now 14 but had chicken pox back in elementary school. Back when I was a kid in the 60s (didn't know I had CF back then) I got it really really bad and missed a whole month of school. Definitely get the vaccine - it might not keep the kid from completely getting it, but it will greatly reduce the severity of the infection. ~Juliet