In-depth phylogenetic analysis of the hemagglutinin gene of influenza A(H3N2) viruses circulating during the 2016-2017 season revealed egg-adaptive mutations of vaccine strains.

Objectives: The authors conducted an in-depth phylogenetic analysis of the hemagglutinin (HA) gene of A(H3N2) identified during the 2016-2017 season to compare the circulating strains with both the egg-derived and cell-derived 2016-2017 candidate vaccine virus (CVV).Methods: 162 HA1 sequences of H3N2 viruses identified during the 2016-2017 season were phylogenetically analyzed and compared with egg-/cell-derived CVV. The predicted vaccine efficacy (pVE) of egg-/cell-derived CVV against field strains was quantified by Pepitope model.Results: All H3N2 belonged to 3C.2a genetic clade, most (80.2%) to 3C.2a1 sub-clade. Several H3N2 variants circulated in the 2016-2017 season. HA sequences of field H3N2 strains displayed greater identity with cell-derived CVV than with egg-derived CVV. The amino acid substitutions in positions 160 and 194 identified in egg-derived CVV affected the pVE, which was lower for egg-derived CVV than for cell-derived CVV.Conclusions: The results suggested that reduced vaccine effectiveness observed in 2016-2017 season was probably due to changes in the HA of vaccine strains acquired upon adaptation in eggs, which are the basis of most manufacturing systems currently used globally. Egg-free vaccine manufacturing systems would be advisable to improve the effectiveness of influenza vaccines.