An international group of scientists was convened by the World Health Organization, the World Organisation for Animal Health and the Food and Agriculture Organization to develop a standard nomenclature system based on the H5 viral gene found in avian influenza viruses.

Phylogenetic analysis was performed using a variety of approaches on all of the publicly available H5 sequences that have evolved from the original H5N1 virus. The initial results showed that the currently circulating H5N1 viruses could be effectively grouped into numerous virus clades (taxonomy groups) based on the phylogenetic characterization and sequence homology of the gene. Based on criteria used to distinguish various groups of the gene, the system formally identified 20 distinct clades of the virus since its inception in early 2008.


As the viruses within the clades continue to evolve, new sublineages will periodically emerge, according to the scientists. Once the sublineages meet the same specific clade definition criteria as the initial clades, they will be designated as separate clades.