This study proposal is a secondary analysis of an existing database to determine influenza vaccine effectiveness in older adults. Currently the effectiveness of the current influenza vaccine in older adults is unknown and the methodology used to determine that vaccine effectiveness has been called into question. This database is an accumulation of three studies that prospectively tested older adults for influenza and verified influenza vaccination status. This data will be used to determine if the current methodology of test-negative controls will actually control for the bias of frailty, will determine vaccine effectiveness in the very old, and will explore if vaccine manufacturing process impacts effectiveness. This study has the potential to modify current vaccine effectiveness study methodology and to modify influenza vaccines recommendations in older adults.