Quantitative microbial risk assessment and cost-benefit analysis of Legionella contamination in the water system of a large hospital in Southern Italy

Water used in healthcare facilities for human consumption can pose a health risk to patients, healthcare workers, and visitors. Directive (EU) 2020/2184, transposed in Italy by Legislative Decree 18/23 promotes a preventive, risk-based approach along the entire water supply chain through the Water Safety Plans (WSPs). Quantitative Microbial Risk Assessment (QMRA), combined with Cost Benefit Analysis (CBA), allows integrated evaluation of health impacts and economic sustainability of control strategies.This study aimed to develop a decision support method to define a hospital water network surveillance strategy in the Apulia region (Italy) by estimating the risk of Legionella infection and the related costs and benefits under different exposure scenarios. All wards were included, comparing two surveillance strategies: total surveillance (SA) of all water endpoints, and reduced surveillance (SRED), with a limited number of sampling points. Using a Bayesian framework and a dose-response model, the probability of infection and the expected number of Legionnaires’ disease cases were estimated for wards with different risk levels. In parallel, CBA evaluated direct costs, including microbiological analyses, point of use filters, and hospitalization days, economic indicators such as Net Benefit (NB), Willingness to Pay (WP), Incremental Cost Effectiveness Ratio (ICER), and Expected Incremental Benefit (EIB). Results showed lower infection probabilities with SA (2.6×10−5–2.95 × 10−7) compared to SRED (3.2×10−5–3.1 × 10−6), with expected cases of 0.4 vs 1.7. However, SRED was more cost-effective (€276,700 vs €422,900), showing positive NB (€149,959) and EIB (€ 348,787) and negative ICER (−9621.7). Even assuming a WTP of €25,000 per prevented case, surveillance costs exceeded benefits.