Vaccine effectiveness in patients admitted for influenza during the 2023-2024 season.

Background and goal: Vaccination against influenza is widespread worldwide, reducing complications associated with infection. However, the impact of vaccination on mortality/ICU admission in hospitalized patients has been little studied.

Material and methods: A retrospective observational study was conducted on 238 patients hospitalized for influenza from October 2023 to January 2024 to evaluate the vaccine's effectiveness in terms of the combined event of ICU admission/mortality during hospitalization. Additionally, the characteristics of vaccinated patients and the existence of bacterial superinfection were analyzed. Cox regression was performed using the SPSS program and the free «R» software.

Results: A total of 238 patients were included. Those vaccinated were older (78.2±8.8 vs 69.97±16.6years; P<.001) and were more likely to have hypertension (82.2% vs 56.2%; P<.001), cardiovascular disease (36.6% vs 24.1%; P=.05), chronic bronchopathy (25.7% vs 8.8%; P=.001), or chronic kidney disease (22.8% vs 8.8%; P=0.005). They had lower levels of CRP (8.39±9.55 vs 11.03±10.75mg/dl; P=.05), procalcitonin (0.62±1.74 vs 1.67±4.57ng/dl; P=.05), and SOFA scores (1.13±0.9 vs 1.39±0.97; P=0.033). 11 patients were admitted to ICU (4.6%) and 11 died (4.6%). Influenza vaccination was associated as a protective factor against ICU admission/mortality in the Cox regression (HR=0.216; 95%CI: 0.062-0.759, P=.017). The presence of bacterial superinfection was similar between vaccinated and unvaccinated patients (63.4% vs 67.9%; P=.556).

Conclusions: Influenza vaccination may reduce the probability of ICU admission or death. This effect is likely due to better control of the immune response. We did not observe any relationship with the risk of presenting bacterial superinfection.