Outbreak of Legionnaires' Disease Linked to Newly Installed Residential Water Heaters, the Netherlands, 2022-2023.

During 2022-2023, two small Legionnaires' disease (LD) clusters (2 and 4 cases) occurred in 2 residential apartment buildings in the Netherlands. All case-patients recently installed a new brand A water heater. Environmental sampling revealed Legionella pneumophila serogroup 1 sequence type 37 in the hot water system of each case-patient's apartment, matching 1 clinical isolate. We conducted a case-control study to evaluate whether brand A water heaters were linked to cases in the 2 clusters. We identified 23 LD case-patients, 21 of whom had a brand A water heater installed <6 months before illness onset. Four cases had a genotypic match between clinical and environmental isolates; none of 31 control-patients had recently installed a water heater. Analyses showed that LD cases were strongly associated with new brand A water heaters (OR 542 [95% CI 24.76-11,854.03]); the manufacturer implemented control measures. Residential water heaters could serve as L. pneumophila transmission sources.