Since 2003 to date. there have been a number of genotypes and clades of avian influenza virus A/H5N1 recognized in avian population in Vietnam. Regarding to genotypes. there are genotype Z and G (namely, VN1-VN9)
and those of clades ie.. classical clade 1 introduced since 2003, clade 2.3.4 from 2007 and clade 2.3.2 in recent years. From isolates collected during 2004-2011, the entire sequence of hemagglutinin HA (H5) (1704-1707bp) for 14 virulent strains of A/H5N1 was obtained by using PCR and sequencing after cloning. Genetic and phylogenetic analysis of HA (H5) revealed that by now in Vietnam. clade 1.1 has newly emerged and coexists with classical clade 1 in Southern: and clade 2.3.2.1 completely replaced clade 2.3.4 circulating since 2007 in Northern Vietnam. Isolates collected after 2008 in Southern (mainly, in Mekong river delta), belong to clade 1.1
and those isolated in Mid of 2011 (in Quang Tri province) to clade 2.3.2.1, highly identical to the human fatal A/Hubei/1/2010(H5N1) (GenBank: CY098758) strain of China. These viruses were grouped to the 2.3.2.1. a newly defined clade by FAD since August. 2011. It is concluded that during 2004-2011, due to introduction and evolution of avian influenza A/H5N 1. there have been a number of clades emerged in Vietnam, with changes in genetic and antigenic properties that may affect protection in immunized poultry with vaccine of clade 1 applied nationwide since 2006.