The effectiveness of influenza vaccination in preventing hospitalisations of elderly individuals in two influenza seasons: a multicentre case-control study, Spain, 2013/14 and 2014/15

Influenza vaccination may limit the impact of influenza in the community. The aim of this study was to assess the effectiveness of influenza vaccination in preventing hospitalisation in individuals aged >= 65 years in Spain. A multicentre case-control study was conducted in 20 Spanish hospitals during 2013/14 and 2014/15. Patients aged >= 65 years who were hospitalised with laboratory-confirmed influenza were matched with controls according to sex, age and date of hospitalisation. Adjusted vaccine effectiveness (VE) was calculated by multivariate conditional logistic regression. A total of 728 cases and 1,826 matched controls were included in the study. Overall VE was 36% (95% confidence interval (CI): 22-47). VE was 51% (95% CI: 15-71) in patients without high-risk medical conditions and 30% (95% CI: 14-44) in patients with them. VE was 39% (95% CI: 20-53) in patients aged 65-79 years and 34% (95% CI: 11-51) in patients aged >= 80 years, and was greater against the influenza A(H1N1) pdm0(9) subtype than the A(H3N2) subtype. Influenza vaccination was effective in preventing hospitalisations of elderly individuals.