The H5N1 virus can be divided into 10 clades, according to the evolutionary position of the HAs defined by the WHO (World Health Organization) in 2008. These clades are numbered 0 to 9 [48, 49] . Clades 0,2 are responsible for all the human infections, resulting in 359 deaths since 2003 [11] . Many clades have not been reported for the last four years, while the dominant H5N1 viruses are concentrated in clade 0,2. Clade 2.1, clade 2.2 and clade 2.3 have further evolved into third-or fourth-order clades, and these newly formed viruses have become geographic strains.