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

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Abstract
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. J. Med. Virol. 70:463–469, 2003.