Nucleotide sequences were manually edited in FASTA format, using BioEdit v7.0.5 [7] , and aligned with CLUSTAL W [8] . Sequence information (GenBank accession number, strain, and year of isolation) for each sample used in this study are available for HA (Table S1) and NA genes (Table S2) , respectively. Pairwise distances were calculated with MEGA v5 [9] . The percentages of identities were calculated by applying the formula 100 -(pairwise distance value x 100). A graph was constructed by plotting the percentage identities in the abscissa (x axis) vs the frequency of each of the calculated pairwise identities in the ordinate (y axis). The graphs were prepared in the R environment, using ggplot2 package (www.r-project.org).