Comparison of the Protective Efficacy of Flublok® Quadrivalent Versus Licensed Inactivated Influenza Vaccine (IIV4) in Healthy, Medically Stable Adults ≥50 Years of Age

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.

In various clinical studies the investigators demonstrated that the immune response against the influenza A viruses is improved as a result of the higher hemagglutinin content. Furthermore, influenza virus disease and hospitalization associated with influenza-related illness in older adults (> 50 years) was considerably reduced (90%) following vaccination with TIV, even though the circulating influenza A strain was antigenically dissimilar to that in the vaccine. However, more recently Skowronski et al. reported that the low influenza vaccine effectiveness in 2012-2013 was not associated with antigenic drift but was instead related to mutations in the egg-adapted H3N2 vaccine strain. Flublok manufactured using recombinant technology does not contain the mutations responsible for the reported lower effectiveness and may thus offer improved protection when mutations such as those described are induced in the process of adapting the influenza virus to growth in eggs.