Recently, the hemagglutinin characteristics, pathogenicity, and antigenic variation of highly pathogenic (HP) H7N9 avian influenza viruses (AIVs) were reported in this journal.1 H7N9 AIVs have been endemic in chicken since their emergence in China in February 20132, and have triggered five epidemics of human infections.3, 4 At first, the 2013 H7N9 viruses were nonpathogenic in chickens. However, some H7N9 viruses transitioned from low to high pathogenicity for chicken during the 5th wave.5, 6 These H7N9 HPAIVs were not only destructive to poultry, but also lethal to humans.