Influenza vaccine effectiveness against laboratory-confirmed influenza in hospitalised adults aged 60 years or older, Valencia Region, Spain, 2017/18 influenza season

Introduction: Influenza immunisation is recommended for elderly people each season. The influenza vaccine effectiveness (IVE) varies annually due to influenza viruses evolving and the vaccine composition. Aim: To estimate, in inpatients >= 60 years old, the 2017/18 trivalent IVE, overall, by vaccine type and by strain. The impact of vaccination in any of the two previous seasons (2016/17 and 2015/16) on current (2017/18) IVE was also explored. Methods: This was a multicentre prospective observational study within the Valencia Hospital Surveillance Network for the Study of Influenza and Respiratory Viruses Disease (VAHNSI, Spain). The test-negative design was applied taking laboratory-confirmed influenza as outcome and vaccination status as main exposure. Information about potential confounders was obtained from clinical registries and/or by interviewing patients; vaccine information was only ascertained by registries. Results: Overall, 2017/18 IVE was 9.9% (95% CI: -15.5 to 29.6%), and specifically, 48.3% (95% CI: 13.5% to 69.1%), -29.9% (95% CI: -79.1% to 5.8%) and 25.7% (95% CI: -8.8% to 49.3%) against A(H1N1) pdm09, A(H3N2) and B/Yamagata lineage, respectively. For the adjuvanted and non-adjuvanted vaccines, overall IVE was 10.0% (95% CI: -24.4% to 34.9%) and 7.8% (95% CI: -23.1% to 31.0%) respectively. Prior vaccination significantly protected against influenza B/Yamagata lineage (IVE: 50.2%; 95% CI: 2.3% to 74.6%) in patients not vaccinated in the current season. For those repeatedly vaccinated against influenza A(H1N1) pdm09, IVE was 46.4% (95% CI: 6.8% to 69.2%). Conclusion: Our data revealed low vaccine effectiveness against influenza in hospitalised patients >= 60 years old in 2017/18. Prior vaccination protected against influenza A(H1N1) pdm09 and B/Yamagata-lineage.