Molecular characterization of influenza B viruses circulating in northern Italy during the 2001-2002 epidemic season

During the 2001-2002 influenza season, virological surveillance highlighted the predominant circulation of B viruses (86% of isolates) in Italy, in contrast to many other countries in Europe and North America where AH3N2 viruses were isolated most frequently, and in contrast to the infrequent isolation of B viruses in Italy during the previous two years. Associated with this predominance of influenza B was the re-emergence of B/Victoria/2/87-lineage viruses, closely related to B viruses prevalent during the 1980s, which are distinct antigenically and genetically from circulating B/Sichuan/379/99-like viruses of the B/Yamagata/16/88 lineage, which predominated in most parts of the world during the last 10 years. Ninety-four viruses isolated in two regions of northern Italy were characterized, 50 by direct sequencing of haemagglutinin (HA). Viruses of both Victoria and Yamagata lineages co-circulated throughout the 12 weeks of the influenza season. The HAs of the Yamagata-lineage viruses were heterogeneous and comprised two sublineages, represented by B/Sichuan/379/99 and B/Harbin/7/94, whereas the Victoria-lineage viruses were more homogeneous and closely related to B/Hong Kong/330/01, the current prototype vaccine strain. The antigenic and genetic characteristics of the viruses correlated with certain epidemiological features. In particular, the low age (<14 years) of individuals infected with B/Hong Kong/330/01-like viruses is likely to reflect the greater susceptibility of the youngest cohort, due to lack of previous exposure to Victoria-lineage viruses, and is consistent with the conclusion that vaccination with a B/Sichuan/379/99-like virus would give poor protection against infection with B/Hong Kong/330/01-like (Victoria-lineage) viruses.