Seasonal flu is a common cause of hospital admission, especially in populations with comorbidities or extreme ages. The objective was to investigate the effectiveness of influenza vaccination in hospitalized laboratory confirmed influenza cases (HLCI). A retrospective case-to-case study of HLCI adults in Catalonia, during the 2017-2018 season was carried out. Differences in means, proportions, factors associated with vaccine effectiveness (VE) and intensive care unit (ICU) were assessed by

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... t, Chi-squared test, logistic regression and multivariate logistic regression, accordingly. A total of 1414 HLCI aged 18 years and older were included in the analysis, 465 (33%) vaccinated, (of these 94% were ≥ 60 yrs.), 804 (56.9%) were men, 61% were type B influenza and 15.1% required ICU admission. An age of 60 or older was associated with lower ICU admission (OR 0.44; 95%CI 0.32-0.61; <0.001). Mean length of hospital stay (LOS) and ICU stay (LICS) did not differ significantly between vaccinated and unvaccinated, although ICU admission proportion did (11.2% in vaccinated vs. 17.1% in unvaccinated). A longer hospital stay was observed in those patients being admitted to ICU being 22.4 (SD 20.3) days vs 11.1(SD14.4) days (p<0,001) for those not admitted to ICU. Being vaccinated avoids ICU admission, aOR of 69% (95%CI 0.49-0.99; 0.04). Considering types A and B jointly, VE to avoid ICU admission was 31% (95%CI 1-52). For type B virus only, VE was 25% (95%CI 18-51). Annual influenza vaccination can reduce the need of admission to ICU in cases of laboratory confirmed influenza virus infections. A shorter mean hospital stay was observed in vaccinated cases, but the result is not statistically significant.