I. To track occurrences of flu-like illness in the study population from November 1, 2011 through April 1, 2012, using a standardized epidemiologic questionnaire. (Observational component) II. To determine the relationship between 25-hydroxy vitamin D3 levels at time of vaccination and its immunologic responses Week 20 (8 weeks [+/- 7 days]) and Week 24 (12 weeks [+/- 7 days]) following administration of the injected seasonal (2012-2013) trivalent influenza vaccine. (Intervention component) III. To track occurrences of flu-like illnesses in the study population from November 2012 through April 2013, using a standardized epidemiologic questionnaire. (Intervention component) IV. To evaluate the association between single-nucleotide polymorphisms (SNPs) and polymorphisms in the deoxyribonucleic acid (DNA) sequence of vitamin-D3 metabolizing enzymes, measures of vitamin-D3 metabolism (24,25 hydroxy [OH] vitamin D3) and response to seasonal (2012-201) trivalent influenza vaccine. (Intervention component)