My 3 year old twin nephews recently had chicken pox and they were vaccinated, however the dose was reasonably mild.  My 4 year old and 2 year old, who have not been vaccinated, were in contact with their cousins, and also the 4 year old was in contact with children at kindy who got chickenpox, and they both did not get it!   Who knows!!  Also, I got pregnant with 2 of my 3 girls whilst using birth control and breastfeeding!!