Co-circulation of the two influenza B lineages during 13 consecutive influenza surveillance seasons in Italy, 2004\u20132017

Simona Puzelli,The Italian Influenza Laboratory Network ,Caterina Rizzo,Concetta Fabiani,Antonino Bella,Paola Affanni,Laura Calzoletti,Annapina Palmieri,Giuseppina Di Mario,Elena Pariani,Angela Di Martino,Maria Chironna,Marzia Facchini,Simone Giannecchini,Maria Rita Castrucci,Isabella Donatelli,Caterina Serra,Barbara Camilloni,Pierlanfranco D’Agaro

doi:10.1186/s12879-019-4621-z

Abstract

BackgroundSince 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.MethodsFrom 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.ResultsCo-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.ConclusionsThis 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.

Highlights

Since 1985, two antigenically distinct lineages of influenza B viruses (Victoria-like and Yamagata-like) have circulated globally
Influenza surveillance in Italy and retrospective data collection Influenza surveillance in Italy is based on a sentinel network of physicians (InfluNet) who report the number of patients with an influenza-like illness (ILI) on a weekly basis and collect respiratory specimens from November to April, for virological analyses
Preliminary analyses are performed on clinical samples at regional level and a representative subset of influenza virus-positive samples and/or virus isolates is sent to the National Influenza Centre (NIC) and subsequently shared with the World Health Organization (WHO)-CC for further antigenic and genetic analyses

Summary

Introduction

Since 1985, two antigenically distinct lineages of influenza B viruses (Victoria-like and Yamagata-like) have circulated globally. 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. Influenza A(H1N1), A(H3N2) and influenza B viruses are responsible for a significant disease burden during seasonal epidemics in humans [1]. Worldwide, these annual epidemics are estimated to cause about 3–5 million cases of severe illness and 290,000–650,000 deaths [2]. In the 1990s, Yamagata-like viruses became predominant worldwide, whereas the Victoria lineage was mainly restricted to East Asia. The two B lineages have been co-circulating worldwide, with variability in terms of geographic distribution and genomic evolution [10,11,12]

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