The effectiveness of the chickenpox vaccine decreases significantly in the first year after vaccination, and also appears lower if the vaccine is administered to children younger than 15 months of age. The vaccine does not always provide protection, and children who have been vaccinated can still contract chickenpox. However, if a child eats properly the likelihood of any complications from naturally acquired chickenpox is quite minimal. The natural infection is far better at providing an immune protective response.
Journal of the American Medical Association February 18, 2004;291(7):851-5.