Inactivated influenza virions are a flexible vaccine platform for eliciting protective antibody responses against neuraminidase.

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 neuraminidase (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 antibody responses against NA. Using a DBA/2J mouse model, we show that the strong infection-induced NA inhibitory (NAI) antibody responses are only achieved by high dose immunizations of inactivated virions, likely due to the 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 showed enhanced NAI antibody responses and improved NA-based protection from a lethal viral challenge while also allowing for the development of natural immunity to the heterotypic challenge virus HA. Second, we combined inactivated virions with recombinant NA protein antigens. These combination vaccines increased NA-based protection following viral challenge and elicited stronger antibody responses against NA than either component alone, especially when the NAs possessed similar antigenicity. Together, these results indicate that inactivated virions are a flexible platform that can be easily combined with protein-based vaccines to improve protective antibody responses against influenza antigens.