Seasonal influenza surveillance: Observational study on the 2017-2018 season with predominant B influenza virus...

Influenza is a common respiratory infectious disease affecting population worldwide yearly. The aim of this work is to describe the 2017-2018 influenza season and how it affected elderly population in Catalonia despite moderate vaccine coverage among this age group.

Influenza surveillance based on a primary care sentinel surveillance, virological indicators systematic sampling of ILI attended and severe influenza confirmed cases (SHLCI) admitted to hospital.Analysis of data by Chi-squared, ANOVA, multiple regression and negative control test or case to case for vaccine effectiveness assessment in primary care and SHLCI respectively.

Moderate-high intensity and early onset season with predominance of influenza B virus (IVB) (63%) followed by an increase of circulation of influenza A virus (IVA). A total of 419 IV from primary care samples. Vaccine effectiveness (VE) in primary care setting was 14% (95%CI: 0-47%). 1306 severe cases (adjusted cumulative incidence 18.54/100,000 inhabitants (95%CI: 17.54-19.55)). The highest proportion of severe cases were in the >64 (65.1%) (aOR 15.70; 95%CI: 12.06-20.46; p < 0.001) followed by 45-64 yo (25.4%) (aOR 6.03; 95%CI: 4.57-7.97). VE in preventing intensive care unit (ICU) admission was 35% (95%CI: 10-54%). Final outcome death while hospitalized occurred in 175 SHLCI cases with a case fatality rate of 13.4%.

2017-2018 influenza season was an unusual epidemic season with an early onset, great predominance of influenza B (Yamagata strain) virus with a high hospitalization rate of severe cases among elderly stressing the need to upgrade vaccine uptake in this age group.