Immunity to influenza A H9N2 viruses induced by infection and vaccination

Avian influenza A H9N2 viruses are widespread among domestic poultry and we re recently isolated from humans with respiratory illness in China. Two ant igenically and genetically distinct groups of H9N2 viruses (G1 and G9) are prevalent in China. To evaluate a strategy for vaccination, we compared G1 and G9 viruses for their relative immunogenicity and cross-protective effic acy. Infection of BALB/c mice with representative viruses of either group p rotected against subsequent challenge with the homologous or heterologous H 9N2 virus in the absence of detectable cross-reactive serum hemagglutinatio n inhibition antibody. Mice injected intramuscularly with inactivated G1 wh ole virus vaccine were completely protected from challenge with either H9N2 virus. In contrast, mice administered inactivated G9 vaccine were only par tially protected against heterologous challenge with the G1 virus. These re sults have implications for the development of human vaccines against H9N2 viruses, a priority for pandemic preparedness.