Distribution of fold-reduction in haemagglutination inhibition (HI) titres relative to the 2012–13 egg-passaged H3N2 strain by nature and location of additional amino acid (AA) mutations present in HA1 antigenic sites of circulating viruses.

HA1 = haemagglutinin 1 protein.Mutations are highlighted in bold in the first row that they are represented in the table.HA1 antigenic site positions [A–E] affected are annotated in bold the first time they appear.“(x n)” following a specified amino acid residue indicates the number of viruses with that specific mutation.a. The 2012–13 egg-passaged H3N2 strain used in haemagglutination inhibition (HI) assay was identical in its HA1 to the egg-adapted A/Victoria/361/2011-IVR-165 high growth reassortant vaccine strain.b. In addition to the 3 AA differences (at positions 156, 186, and 219) present in the egg-passaged H3N2 strain used in the HI assay and the egg-adapted A/Victoria/361/2011 IVR-165 high growth reassortant vaccine strain.