Throughout most of the last decade, B/Yamagata/16/88-lineage influenza viruses were predominant among the B isolates circulating worldwide, whereas B/Victoria/2/87-lineage viruses were isolated infrequently and restricted geographically to eastern Asia. During the 2001-02 influenza season, B/Victoria/2/87-lineage viruses re-emerged in North America and Europe and spread worldwide. Virological surveillance in Italy during that season showed wide circulation of influenza B viruses, of which most were antigenically related to the B/Sichuan/379/99 (Yamagata-lineage) vaccine strain, together with a smaller number of B viruses antigenically similar to B/HongKong/330/01, a recent B/Victoria/2/87-lineage antigenic variant. In the subsequent 2002-03 epidemic season, B viruses with a Victoria-lineage hemagglutinin (HA), more closely related to that of B/Shandong/7/97, were isolated exclusively. Similar strains have continued to predominate among the few B viruses isolated in Italy during last season (2003-04), although most influenza B viruses, isolated sporadically elsewhere in Europe, again belong to the Yamagata-lineage. In the present study, 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 HA genes, 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. The frequency of reassortment between HA and NA of distinct lineages and sublineages highlights again the importance of detailed molecular analyses of both surface glycoproteins in understanding the evolution of influenza B viruses.
2004 Wiley-Liss, Inc.