Multi-Year Study of Fluzone High-Dose Influenza Vaccine Compared With Fluzone® Vaccine in Adults Aged 65 Years and Older

The aim of this study is to determine the efficacy of Fluzone High Dose vaccine and that of Fluzone® vaccine in the elderly. Primary objective: To measure the efficacy of the vaccine, defined as the prevention of laboratory-confirmed influenza caused by viral types/subtypes that are antigenically similar to those contained in the respective annual vaccine formulations. Secondary objectives: * To compare the clinical efficacy of Fluzone High Dose vaccine with that of Fluzone® vaccine in elderly adults, with respect to laboratory-confirmed influenza illness caused by any type or subtype of influenza virus. * To compare the clinical efficacy of Fluzone High Dose vaccine with that of Fluzone® vaccine in elderly adults, in preventing culture-confirmed influenza illness caused by viral types/subtypes antigenically similar to those contained in the respective annual vaccine formulations.