I agree with Rick. The most difficult thing is to get a kid that has CAH healthy once some bug sets in. My only goal in life is to keep my kids healthy 100% of the time (still striving for that).
Ex. My son never has a cold. It always escalates to a virus, or bronchitis, even pneumonia.
My son did have a reaction to the flu shot, once. He just had a vey very very mild case the evening he had the shot. He never had a reaction to the chicken pox vaccine, nor MMR.
I have two other children that are also fine with their vaccines.
It is true that the steroids a CAH person is on is simply replaceing what the body does not manufacture. Might be a problem if the person is being grossly oversuppressed.