Doctors (endo and pediatrician) were both concerned and did not want my daughter to catch chicken pox.  This was a year or two before the vaccine was approved here in the US.  Because she was exposed to so many other kids with it at school, they gave me a prescription for Zovrax and it was wonderful.  It must be given within 24 hrs of the first pox.  It caused her to have a very mild case, but the pediatrician assured me enough of a case to have immunity.  We did stress dose her at the time.  Now that they have the vaccine I would recommend it to everyone - with or without CAH.  Why take any chances?