Genetic and subunit vaccines based on the stem domain of the equine influenza hemagglutinin provide homosubtypic protection against heterologous strains

Sera from mice inoculated with these immunogens in different combinations and regimes presented reactivity in vitro against highly divergent influenza virus strains belonging to phylogenetic groups 1 and 2 (H1 and H3 subtypes, respectively), and conferred robust protection against a lethal challenge with both the homologous equine strain (100%) and the homosubtypic human strain A/Victoria/3/75 (H3N2) (70-100%). Animals vaccinated with the same antigens but challenged with the human strain A/PR/8/34 (H1N1), belonging to the phylogenetic group 1, were not protected (0-33%). Combination of protein and DNA immunogens showed higher reactivity to non-homologous strains than protein alone, although all vaccines were permissive for lung infection.