Tap water as the source of a Legionnaires' disease outbreak spread to several residential buildings and one hospital, Finland, 2020 to 2021.

In Finland, all microbiology laboratories notify findings and physicians notify Legionnaires' disease (LD) cases to the National Infectious Disease Register. All cases are interviewed, and water samples obtained from potential places of exposure. isolates from humans and water are compared by whole genome sequencing (WGS). In March 2021, serogroup 1 (Lp 1) pneumonia cases increased in one Finnish city (120,000 inhabitants) where single LD cases are detected annually. We identified 12 LD cases, nine living in different residential buildings and three nosocomial, linked by identical human and/or water isolates. Three of these cases were from January 2020, October 2020 and February 2021 and identified retrospectively. Eleven were diagnosed by urinary antigen test, 10 by PCR and five by culture; age ranged between 52 and 85 years, and 10 had underlying diseases. Nine of 12 homes of LD cases and 15 of 26 water samples from the hospital were positive for Lp 1, with concentrations up to 640,000 cfu/L. Water samples from regional storage tanks were negative. Positivity in homes and the hospital suggested inadequate maintenance measures. Enhanced surveillance combined with WGS was crucial in detecting this unusual LD outbreak related to domestic and hospital water systems.