Low 2012–13 Influenza Vaccine Effectiveness Associated with Mutation in the Egg-Adapted H3N2 Vaccine Strain Not Antigenic Drift in Circulating Viruses
One HA1 monomer is shown with five previously defined antigenic site residues of A–E colored in light green, dark green, light blue, dark blue and purple, respectively, mapped onto a related crystal structure (A/X-31(H3N2), PDB, 1HGG)  using PyMOL . The most prevalent antigenic site amino acid differences between circulating clade 3C viruses in Canada relative to the egg-adapted A/Victoria/361/2011 IVR-165 vaccine reassortant strain are shown in red and labelled with coloured font representing their antigenic sites, viewed from the front (A) or side (B). Three amino acid differences (Q156H, V186G and Y219S) are owing to mutation in the egg-adapted IVR-165 vaccine strain rather than circulating viruses which instead share identity with the MDCK-passaged WHO reference prototype at these positions. RBS indicates approximate location of the receptor-binding site.