Phylogenetic analysis of the haemagglutinin (HA) gene of the 36 A/H1N1pdm09 strains characterised in Apulia in the 2017-2018 season (black dots). The phylogenetic tree was constructed using Mega 7 software applying the Neighbor Joining algorithm using Kimura-2 parameter-corrected distances. Amino acid substitutions defining specific genetic clusters are shown at nodes. Genetic clades are shown to the right of the tree, and the scale bar represents nucleotide substitutions per site. For each sequence deposited in GISAID, the accession number is indicated in parentheses. Strain-specific amino acid substitutions are indicated after strain names (in bold font), as are clinical definitions (ILI, influenza-like illness; SARI, severe acute respiratory illness; ICU-ARDS, cases with acute respiratory distress syndrome requiring admission to an intensive-care unit) and, if vaccinated, the type of vaccine (TIV, trivalent vaccine; aTIV, adjuvanted trivalent vaccine; QIV, quadrivalent vaccine).