Annual flu vaccination is the most effective way to prevent the disease and its complications. Vaccine effectiveness (EV) varies from season to season, requiring annual re-evaluation. The objective of this study was to estimate the preliminary effectiveness of the influenza vaccine until epidemiological week 4 of the 2019–2020 season, in patients admitted to a tertiary-level hospital. Method: We conducted a case-control study at University General Hospital, Alicante, Spain, during the 2019–2020

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... season. We included all patients hospitalized with influenza confirmed by laboratory test (ie, PCR positive for influenza) during the period between epidemiological week 40 of 2019 and epidemiological week 4 of 2020. These were considered cases, and those with clinical suspicion of influenza and negative RT-PCR were considered controls. Vaccination coverage was calculated in cases and in controls, determining the odds ratio. We calculated the vaccine effectiveness (VE) and its 95% confidence interval using the following formula: VE = (1 − odds ratio) ×100. Result: We included 545 patients: 61 cases and 484 controls. The overall EV for influenza cases prevention was 40.7% (95% CI, −17.1 to 70.1), and for those >1 year of age, the overall EV was 56.9% (95% CI, 13.9–78.5). Conclusion: The 2019–2020 Influenza vaccine was effective in preventing influenza cases in patients admitted up to week 4 of the 2019–2020 season. These results are preliminary and may vary; they should be re-evaluated at the end of the season.Funding: NoneDisclosures: None