Phylogenetic analysis of H9N2 subtype avian influenza viruses isolated from immunized chicken flocks and protective efficacy of F strain inactivated vaccine.

To investigate the phylogenesis and antigenic relationship of H9N2 avian influenza viruses(AIV),the hemagglutinin(HA) genes of 9 isolates from immunized chicken flocks in China during 2009 to 2010 were sequenced and analyzed.The HA genes were 1,683 nt in length,sharing 90.7% to 98.9% nucleotide acid homology and 92.2% to 98.8% amino acid homology.There were high sequence similarities between these isolates and the vaccine strain(F),with 91.6%-99.6% identities at nucleotide acid level,and 92.9% to 99.3% identities at amino acid level.All isolates belonged to the Beijing/1/94-like lineage and could be divided into three genotypes.Commercial subtype H9 inactivated vaccine(F strain) could stimulate the high antibody titers in SPF chickens and markedly reduced the virus shedding after challenge with H9N2 AIV isolates.