Legionella Pneumonia in the Modern Era: Clinical Features and Predictors of Mortality

BACKGROUND: Legionella pneumonia is an important cause of severe pneumonia, yet contemporary data describing its clinical phenotype, diagnostic patterns, and outcomes in the modern era are limited. METHODS: We conducted a retrospective multicenter cohort study of adults with laboratory confirmed Legionella pneumonia across an integrated health system between January 2019 and September 2025. The primary outcome was 30-day mortality. Severe disease was defined as having a requirement for high-flow nasal cannula, noninvasive ventilation, mechanical ventilation, or extracorporeal membrane oxygenation (ECMO). Univariate and multivariable logistic regression were used to identify predictors of mortality and severe disease. RESULTS: There were 344 patients with laboratory-confirmed Legionella pneumonia during the study period. The median age was 66.6 years, and 45.1% were immunocompromised. Most patients required hospital admission (94.2%), with 36.1% admitted to the intensive care unit; 22.7% required mechanical ventilation and 1.5% extracorporeal membrane oxygenation. Thirty- and 90-day mortality were 11.9% and 16.6%, respectively. In multivariable analysis, cirrhosis (OR 10.2, 95% CI 2.15-48.3, p = 0.003), immunocompromised status (OR 2.24, 95% CI 1.05-4.77, p = 0.036), age (OR: 1.03, 95% CI: 1.00-1.06, p = 0.031), and lymphopenia at presentation (OR 2.09, 95% CI 1.02-4.24, p = 0.043) were independently associated with increased 30-day mortality. Among patients who were PCR or culture positive, urinary antigen testing was positive in only 25.6%. CONCLUSIONS: Legionella pneumonia was associated with substantial short-term mortality. Cirrhosis, age, immunocompromised status, and lymphopenia at presentation emerged as independent predictors of 30-day mortality, underscoring the importance of host vulnerability in determining outcomes.