Novel correlates of protection against pandemic H1N1 influenza A virus infection.

Influenza viruses remain a severe threat to human health, causing up to 650,000 deaths annually. Seasonal influenza virus vaccines can prevent infection, but are rendered ineffective by antigenic drift. To provide improved protection from infection, novel influenza virus vaccines that target the conserved epitopes of influenza viruses, specifically those in the hemagglutinin stalk and neuraminidase, are currently being developed. Antibodies against the hemagglutinin stalk confer protection in animal studies. However, no data exist on natural infections in humans, and these antibodies do not show activity in the hemagglutination inhibition assay, the hemagglutination inhibition titer being the current correlate of protection against influenza virus infection. While previous studies have investigated the protective effect of cellular immune responses and neuraminidase-inhibiting antibodies, additional serological correlates of protection from infection could aid the development of broadly protective or universal influenza virus vaccines. To address this gap, we performed a household transmission study to identify alternative correlates of protection from infection and disease in naturally exposed individuals. Using this study, we determined 50% protective titers and levels for hemagglutination inhibition, full-length hemagglutinin, neuraminidase and hemagglutinin stalk-specific antibodies. Further, we found that hemagglutinin stalk antibodies independently correlated with protection from influenza virus infection.