I. To examine the effect of 25-hydroxy vitamin D3 levels on the immunologic response (antibody responses week 4 [4 weeks (+/- 7 days)] post-vaccination) to the injectable seasonal (2011-2012) trivalent influenza vaccine among Roswell Park Cancer Institute (RPCI) employees. (Observational component) II. To examine the effect of vitamin D3 (cholecalciferol) supplementation on the immunologic response (antibody response Week 16 [4 weeks (+/- 7 days)] post-vaccination) to the injectable seasonal (2012-2013) trivalent influenza vaccine among RPCI employees. (Intervention component)