 The goal of this study is to establish that Flublok Quadrivalent is non-inferior to fully licensed traditional approval status quadrivalent inactivated influenza vaccine IIV4 in protecting against laboratory-confirmed clinical influenza disease in the 50 year age population. Real-time Polymerase Chain Reaction rtPCR will be used to confirm influenza infection and to type the strains involved, as molecular methodologies have been demonstrated to be more sensitive than other more traditional methodologies, e.g. culture. For rtPCR-positive clinical samples, reserved aliquots will be processed for culture, so that antigenic similarity to the HA present in study vaccines can be tested. 