Vaccination status as a determinant of hospitalization in influenza: Insights from emergency department data

This study aims to investigate the effect of seasonal influenza vaccination on hospitalization rates among patients presenting to the emergency department with influenza-like illness.

A retrospective, single-center observational study was conducted, involving adult patients with influenza (ICD-10 codes J10 and J11) diagnosed in the emergency department between May 2024 and April 2025. Clinical and demographic information was collected from electronic records, and vaccination status was confirmed through follow-up phone calls. To tackle the "zero event" problem-no hospitalizations among vaccinated individuals-advanced statistical modeling was employed, including standard logistic regression, and Bayesian logistic regression using Markov Chain Monte Carlo (MCMC) simulations. Odds ratios (OR) and 95% Highest Density Intervals (HDI) were calculated to assess the effectiveness of vaccination.

A total of 878 patients were enrolled: 3.3% (n = 29) received vaccinations, while 2.7% (n = 24) required hospitalization. None of the vaccine recipients were hospitalized. Standard logistic regression indicated that age was a significant indicator of hospitalization. Furthermore, Bayesian logistic regression followed, which confirmed vaccination's statistically significant protective effect. The OR for vaccination was 0.526 (95% HDI: 0.336-0.739), indicating a 47% reduction in hospitalization risk among vaccinated individuals.

Seasonal influenza vaccination was significantly associated with a lower risk of hospitalization in patients presenting with influenza-like illness to the emergency department. These findings support public health initiatives to enhance influenza vaccine coverage, particularly for the elderly.