Phylogenetic analysis of the surface glycoprotein genes of human type B Italian influenza isolates after the reemergence in 2001 of B/Victoria/2/87-lineage viruses

Two major lineages of influenza B viruses have circulated in humans, the B/Yamagata/16/88 (Yam88)-lineage and the B/Victoria/2/87 (Vic87)-lineage. Throughout most of the last decade, Yam88-lineage viruses predominated. During the 2001–2002 influenza season, however, Vic87-lineage viruses emerged outside Asia and spread worldwide. Virological surveillance in Italy during this season highlighted a predominance of influenza B viruses, most of which were antigenically related to the B/Sichuan/379/99 vaccine strain, belonging to the Yam88-lineage, together with a smaller number of B viruses antigenically similar to a recent Vic87-lineage variant B/Hong Kong/330/01. Vic87-lineage viruses were exclusively isolated in the subsequent 2002–2003 epidemic season. Phylogenetic analyses of the hemagglutinin (HA) and neuraminidase (NA) genes of representative Italian B isolates from the 2001–2002 and 2002–2003 seasons showed that, during the first season, the NA as well as the HA genes separated into two distinct clades, the Yam88- and the Vic87-lineages and showed no evidence of reassortment. On the contrary, all the B viruses isolated in the 2002–2003 epidemic season were “Vic87 HA–Yam88 NA” reassortants similar to those isolated in other parts of the world, showing that these reassortants became well-established in the human population.