Heterologous HA DNA vaccine prime--inactivated influenza vaccine boost is more effective than using DNA or inactivated vaccine alone in eliciting antibody responses against H1 or H3 serotype influenza viruses.

The trivalent inactivated vaccine (TIV) is used to prevent seasonal influenza virus infection in humans, however, the immunogenicity of this vaccine may be influenced by the priming effect of previous influenza vaccinations or exposure to antigenically related influenza viruses. The current study examines the immunogenicity of a clinically licensed TIV in rabbits naïve to influenza antigens. Animals were immunized with either the licensed TIV, a bivalent (H1 and H3) HA DNA vaccine or the combination of both. Temporal and peak level serum anti-influenza virus IgG responses were determined by enzyme-linked immunosorbent assay (ELISA). Functional antibody responses were measured by hemagglutination inhibition and microneutralization against either A/NewCaledonia//20/99 (H1N1) or A/Panama/2007/99 (H3N2) influenza viruses. Our results demonstrate that the immunogenicity of the TIV is low in sero-negative animals. More significantly, the heterologous DNA prime-TIV boost regimen was more immunogenic than the homologous prime-boost using either TIV or DNA vaccines alone. This finding justifies further investigation of HA DNA vaccines as a priming immunogen for the next generation of vaccines against seasonal or pandemic influenza virus infections.