Figure 1.

Hemagglutinin (HA) phylogenetic groups, circulation history, and imprinting patterns. (A) Phylogenetic tree of the 18 known influenza A virus (IAV) HA subtypes and the two HA groups they fall into. Seasonal human subtypes are depicted in larger font. (B) Circulation history of seasonal IAV subtypes, with group 1 strains in shades of blue and group 2 H3N2 in red. (C) Reconstruction of the percentage of each birth year cohort with initial IAV infection with each subtype. The color scheme follows B with the addition of gray, which represents the percentage of infants who remained naive to both H1N1 and H3N2 as of 2015. (B, C, From Gostic et al. 2016; adapted, with permission, from The American Association for the Advancement of Science © 2016.)