Younger age at vaccination may increase risk of varicella vaccine failure.

To determine vaccine effectiveness (VE), a varicella outbreak in a highly vaccinated day-care center (DCC) population in Pennsylvania was investigated. In Pennsylvania, proof of immunity is required for children >or=12 months old for DCC enrollment. Questionnaires were administered to parents of children who had attended the DCC continuously during the study period (1 November 1999-9 April 2000) to determine history of varicella disease or vaccination and for information about any recent rash illnesses.