Nudging healthcare workers: assessing the impact of pre-booked appointments on influenza vaccination uptake

Background: Influenza vaccination coverage among healthcare workers in Italy remains low compared with international benchmarks. Evidence on effective and scalable interventions in hospital settings is limited. Methods: We conducted a clustered quasi-experimental study in a large Italian university hospital, comparing the 2024/25 campaign to 2023/24. Hospital cost centers (n = 277) were non-randomly allocated to intervention (personalized letter with a pre-scheduled on-site appointment; 130 centers, 2,967 healthcare workers) or control (standard information; 147 centers, 1,577 healthcare workers). Administrative records provided uptake. We estimated Difference-in-Differences models at the cost-center level, weighting by center size and clustering standard errors at cost-center level, with subgroup analyzes by profession, gender, and age. Results: Overall coverage increased from 16.0% in 2023/24 to 25.2% in 2024/25. The DiD analysis indicated a significant effect of invitation letters (+4.0 percentage points). Stratification showed heterogeneous responses: the intervention was particularly effective among nurses, female workers, and mid-aged staff, while no effect was observed among physicians, the youngest, and the oldest age groups. Conclusion: Personalized invitation letters with pre-scheduled appointments represent a simple, scalable, and resource-efficient strategy to increase influenza vaccination uptake among HCWs. However, the effect was not homogeneous across subgroups, highlighting the importance of tailoring communication strategies to different professional and demographic profiles.