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

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.

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.

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%.

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.