Association Between the Introduction of Pediatric Influenza Vaccination and Influenza Diagnoses in Primary Care and Hospitalizations: An Interrupted Time Series Study

Introduction: Influenza has a major impact on public health. The best way to prevent it is through vaccination. In Catalonia, influenza vaccination has been recommended for children aged 6 to 59 months since the 2023–24 season. Objective: To assess the association between the implementation of this vaccination program and changes in influenza diagnoses in primary care and influenza-related hospitalizations in all age groups. Materials and Methods: Quasi-experimental study with interrupted time series (ITS) analysis. All influenza diagnoses made in primary care (PC) and all influenza-related hospitalizations in the Central Catalonia health region between October 2018 and August 2025 were included. The monthly aggregated cases were analyzed using segmented negative binomial regression models that accounted for temporal trends, the onset of COVID-19, and the introduction of systematic pediatric influenza vaccination. Results: A total of 6804 influenza diagnoses made in PC and 3252 hospitalizations for influenza were analyzed. A statistically significant decrease was observed in the percentage of influenza diagnoses in PC in the 2–4 (13.5% vs. 10.6%) and 5–14 (26.1% vs. 16.3%) age groups. In the ITS analysis conducted in primary care (PC) settings, the vaccination period was significantly associated with a 13% reduction in expected influenza cases among individuals aged 15–64 years (RR 0.87 [0.78; 0.99]). After sensitivity analysis, these results were no longer statistically significant. The ITS analysis in the hospital setting has not shown a significant reduction in expected influenza cases or in expected admissions. Conclusions: Systematic influenza vaccination in children aged 6 to 59 months has not been shown to be associated with a reduction in influenza cases in primary care or hospitals settings during the early stages of implementation of the new vaccination program.