Modernizing the original influenza vaccine to enhance protective antibody responses against neuraminidase antigens

Most seasonal influenza vaccines are produced using hemagglutinin (HA) surface antigens from inactivated virions. However, virions are thought to be a suboptimal source for the less abundant NA surface antigen, which is also protective against severe disease. Here, we demonstrate that inactivated influenza virions are compatible with two modern approaches for improving protective NA antibody responses. Using a DBA/2J mouse model, we confirmed that the strong infection-induced NA inhibitory (NAI) antibody responses are only achieved by high dose immunizations of inactivated virions, likely due to low viral NA content. Based on this observation, we first produced virions with higher NA content by using reverse genetics to exchange the viral internal gene segments. Single immunizations with these inactivated virions enhanced NAI antibody responses, improved NA-based protection from a lethal viral challenge and allowed the development of natural immunity to the heterotypic challenge virus HA. Second, we combined inactivated virions with recombinant NA proteins. These mixtures increased NA-based protection following viral challenge and elicited stronger NA antibody responses than either component alone, especially when the NAs were homologous. Together, these results indicate that viral- and protein-based vaccines can be combined in a single syringe to improve protective antibody responses to influenza antigens.