The population will be employees who were previously evaluated in the pre-employment phase by the Occupational Health Clinic (OHC) of Colorado Children's Hospital (CHC) from 2004 until 2009 and were found to be seronegative for varicella antibody at that time and were thus vaccinated per CHC protocol. Additionally, a matching number of comparator subjects who were positive for varicella antibody at the pre-employment phase will be selected from the same pool (2004-2009).