Clade Designation Criteria


  1. Maintain previously designated clade numbers when possible (i.e., Clade 2.2 remains 2.2 and Clade 1 remains 1)
  2. New clade designations based on phylogenetic tree topology derived from a large tree constructed with 884 sequences.
    • H5N1 progenitors (closest to gs/Guangdong/1/96) designated as Clade 0
    • Subsequent clades numbered starting from Clade 3 (i.e., Clades 3-9)
    • Clades designated by the presence of a distinct common node shared by at least 4 isolates
    • Subclades/sub-subclades designated as a single clade evolves into more than one distinct lineage (based on sharing of a common node)
  3. Average percentage pairwise distances between and within clades (using the Kimura 2-parameter):
    • Distinct clades should have ≥ 1.5% average distances between other clades
    • Distinct clades should have ≤ 1.5% average distances within the clade (may be slightly higher in clades that have highly evolved outliers (i.e., ck/Shanxi/2/06 in Clade 7)
  4. Bootstrap support for clade defining node (based on 1000 replicates)
    • > 60 at clade defining node
  5. Antigenic properties as measured by the hemagglutination inhibition assay should be used as a correlate of clade designation when the data is available.


(Courtesy of the WHO/OIE/FAO H5N1 Evolution Working Group)