Fig. 3

Phylogenetic analysis of the HA1 nucleotide sequences from influenza B/Yamagata (a) and B/Victoria (b) lineage viruses detected in Italy between 2004 and 2017. Seventy-five HA1 sequences of B/Yamagata (a) and 48 of B/Victoria (b) viruses were compared with those from the WHO-recommended vaccine strains for the northern hemisphere (in red, bold font) and other reference strains previously reported by WHO-CC (in black, bold font). The strains identified in Italy are colored according to their respective season, as illustrated in the legend. The trees were constructed using the Neighbor-joining method, with Kimura-2 parameter-corrected distances. Amino acid substitutions defining specific genetic clusters are indicated at nodes. The scale bar represents the nucleotide substitutions per site