Phylogenetic analyses of the HA and neuraminidase (NA) genes of representative B strains, isolated throughout Italy during 2001–04, showed that during the first influenza season the NA genes, as well as the HAgenes, separated into the two distinct clades, the Yamagata- and Victoria lineages, and showed no evidence of genetic reassortment. On the contrary, all the B viruses isolated in the 2002–03 and most of those isolated in the 2003–04 epidemic season were ‘‘Victoria HA–Yamagata NA’’ reassortants similar to those isolated in other parts of the world, showing that these reassortants became established in the human population.
