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Co-circulation of the two influenza B lineages during 13 consecutive influenza surveillance seasons in Italy, 2004–2017 Open Access

  • Autori: Puzelli, S.; Di Martino, A.; Facchini, M.; Fabiani, C.; Calzoletti, L.; Di Mario, G.; Palmieri, A.; Affanni, P.; Camilloni, B.; Chironna, M.; D'Agaro, P.; Giannecchini, S.; Pariani, E.; Serra, C.; Rizzo, C.; Bella, A.; Donatelli, I.; Castrucci, M.; Ansaldi, F.; Arvia, R.; Azzi, A.; Bagnarelli, P.; Baldanti, F.; Capobianchi, M.; Castaldi, S.; Colucci, M.; Galli, C.; Ghisetti, V.; Orsi, A.; Pagani, E.; Palù, G.; Sanguinetti, M.; Smeraglia, R.; Tramuto, F.; Vitale, F.
  • Anno di pubblicazione: 2019
  • Tipologia: Articolo in rivista
  • OA Link:


Background: Since 1985, two antigenically distinct lineages of influenza B viruses (Victoria-like and Yamagata-like) have circulated globally. Trivalent seasonal influenza vaccines contain two circulating influenza A strains but a single B strain and thus provide limited immunity against circulating B strains of the lineage not included in the vaccine. In this study, we describe the characteristics of influenza B viruses that caused respiratory illness in the population in Italy over 13 consecutive seasons of virological surveillance, and the match between the predominant influenza B lineage and the vaccine B lineage, in each season. Methods: From 2004 to 2017, 26,886 laboratory-confirmed influenza cases were registered in Italy, of which 18.7% were type B. Among them, the lineage of 2465 strains (49%) was retrieved or characterized in this study by a real- time RT-PCR assay and/or sequencing of the hemagglutinin (HA) gene. Results: Co-circulation of both B lineages was observed each season, although in different proportions every year. Overall, viruses of B/Victoria and B/Yamagata lineages caused 53.3 and 46.7% of influenza B infections, respectively. A higher proportion of infections with both lineages was detected in children, and there was a declining frequency of B/Victoria detections with age. A mismatch between the vaccine and the predominant influenza B lineage occurred in eight out of thirteen influenza seasons under study. Considering the seasons when B accounted for > 20% of all laboratory-confirmed influenza cases, a mismatch was observed in four out of six seasons. Phylogenetic analysis of the HA1 domain confirmed the co-circulation of both lineages and revealed a mixed circulation of distinct evolutionary viral variants, with different levels of match to the vaccine strains. Conclusions: This study contributes to the understanding of the circulation of influenza B viruses in Italy. We found a continuous co-circulation of both B lineages in the period 2004–2017, and determined that children were particularly vulnerable to Victoria-lineage influenza B virus infections. An influenza B lineage mismatch with the trivalent vaccine occurred in about two-thirds of cases.