Inference of Genotype–Phenotype Relationships in the Antigenic Evolution of Human Influenza A (H3N2) Viruses
Branch lengths represent antigenic distances (maximum of up- and down-weights for each branch) inferred from a maximum likelihood tree of 258 hemagglutinin sequences of seasonal influenza A (H3N2) virus isolates and serological data. (A) Colored edges show antigenic type transitions, with internal branches with high average antigenic weights (≥1.0 antigenic units). Gray-blue edges represent high weight branches leading to a subtree with three isolates or less, representing low abundance types. (B) Isolates are color-coded by antigenic clusters according to Smith et al. (2004). Three isolates (A/Christchurch/4/85, A/Hong Kong/34/90 and A/Netherlands/172/96) are only present as antisera and were not assigned a cluster label.