To the editor:
The WHO/OIE/FAO H5N1 Evolution Working Group has previously proposed a nomenclature system designed to classify the A/goose/Guangdong/1/1996 lineage of Eurasian highly pathogenic H5N1 avian influenza viruses. To date the nomenclature system has identified 10 distinct clades of viruses based primarily on the phylogenetic characterization and sequence homology of the hemagglutinin (HA) gene. 1 Clades were strictly defined using criteria based on sharing of a common node and monophyletic grouping with a bootstrap value of ≥60 at the clade defining node (after 1000 neighbor-joining bootstrap replicates) and maintaining average percentage pairwise nucleotide distances between and within clades of >1·5% and <1·5% respectively. However, as viruses within these clades continue to evolve, new H5N1 clades (defined by the same specific criteria) can be expected to periodically emerge from within established clades. These new clades are defined as second (or third, etc.) order clades and assigned a numerical ‘address’ which links them to their original clade. In this progressive system, the WHO/OIE/FAO H5N1 Evolution Working Group examines the available data and updates the associated phylogenetic trees and clade nomenclature on an ongoing basis.