High Public-Health Impact in an Influenza-B-Mismatch Season in Southern Italy, 2017-2018

Abstract EN

Background.

Yearly influenza epidemics have considerable effects on public health worldwide.

The 2017-2018 influenza season in Italy was of greater severity than previous seasons.

The aim of this study was to describe the 2017-2018 influenza season in Southern Italy and the molecular characteristics of the circulating viral strains.

Methods.

The incidence of influenza-like illness (ILI) was analysed.

Nasopharyngeal swabs collected from patients with ILI from week 46/2017 to week 17/2018 were tested to identify influenza A viruses (IAV) and influenza B viruses (IBV).

Sequencing and phylogenetic analysis of haemagglutinin genes were also performed on 73 positive samples (35 IBV, 36 IAV H1, and 2 IAV H3 strains).

Results.

During the 2017-2018 season, the peak incidence was 14.32 cases per 1,000 inhabitants.

IBV strains were identified in 71.0% of cases.

The 35 characterised IBV strains belonged to Yamagata lineage clade 3, the 36 A/H1N1pdm09 strains clustered with the genetic subgroup 6B.1, and the 2 A/H3N2 strains clustered with the genetic subgroup 3C.2a.

Intensive-care unit (ICU) admission was required in 50 cases of acute respiratory distress syndrome (ARDS).

Among the >64-year age group, 18 out of 26 ICU-ARDS cases (69.2%) were caused by IBV, and 14 of these (77.8%) were B/Yamagata lineage.

Conclusions.

The 2017-2018 influenza season was one of the most severe in a decade in Southern Italy.

IBV mismatch between the trivalent vaccine and the circulating strains occurred.

The high number of ICU-ARDS cases caused by B/Yamagata strains in the >64-year age group suggests that further data on the effectiveness of the available influenza vaccines are needed to determine the best way to protect the elderly against both IBV lineages.