Effectiveness of chlorine treatment of hospital water systems in reducing the incidence of nosocomial legionellosis: a quasi-experimental study

Objective: Data demonstrating a reduction in the incidence of nosocomial legionellosis following chlorine treatment are limited. Therefore, we aimed to evaluate the effectiveness of chlorine treatment by comparing the incidence of nosocomial legionellosis before and after the installation of a chlorine concentration control system. Design: A quasi-experimental study. Setting: A 2,700-bed tertiary care hospital in Seoul, South Korea. Methods: An automatic chlorine concentration control system was sequentially installed in the hospital’s water supply networks across multiple buildings between April 2019 and February 2020. The preintervention period was defined as January 2011 through March 2019, and the postintervention period as May 2020 through November 2025. Incidence rates of nosocomial legionellosis were compared between the two periods using exact Poisson methods. Results: The incidence of nosocomial legionellosis was found to be 0.402 per 100,000 patient-days during the preintervention period and 0.181 per 100,000 patient-days during the postintervention period. The incidence rate ratio for legionellosis during the postintervention period compared with the preintervention period was 0.45 (95% confidence interval, 0.18–1.01; P = .043). Conclusions: The implementation of chlorine treatment in the hospital water distribution system was associated with a significant reduction in the incidence of nosocomial legionellosis. This finding provides long-term, real-world evidence supporting chlorine treatment as an effective water management strategy for preventing healthcare-associated legionellosis.