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

• Main Authors: Simona Puzelli, Angela Di Martino, Marzia Facchini, Concetta Fabiani, Laura Calzoletti, Giuseppina Di Mario, Annapina Palmieri, Paola Affanni, Barbara Camilloni, Maria Chironna, Pierlanfranco D’Agaro, Simone Giannecchini, Elena Pariani, Caterina Serra, Caterina Rizzo, Antonino Bella, Isabella Donatelli, Maria Rita Castrucci, the Italian Influenza Laboratory Network
• Format: Article
• Language: English
• Published: BMC 2019-11-01
• Series: BMC Infectious Diseases
• Subjects: Influenza virological surveillance; Influenza B virus; Victoria lineage; Yamagata lineage; Vaccine match; Italy
• Online Access: http://link.springer.com/article/10.1186/s12879-019-4621-z

Abstract 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.