Effectiveness of Annual Flu Vaccination Until Week Four of the 2019–2020 Season

Background: 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 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: None Disclosures: None