Seasonal influenza-associated intensive care unit admission and death in tropical Singapore, 2011-2015

Background: Seasonal influenza can cause severe illness leading to intensive care unit (ICU) admission and death. Objective: To define the clinical and epidemiological features of severe seasonal influenza infection and factors associated with mortality. Study design: A retrospective review was conducted on all patients with laboratory-confirmed influenza infection who were either admitted into the ICU or died in the two largest tertiary hospitals in Singapore from 2011-2015. Results: Of 520 patients included in our study, 423 (81.3%) had influenza A infection and the rest with influenza B. Of patients with influenza A infection, 70.0% (296/423) were subtyped, of whom 24.0% (71/296) had A/H1N1pdm2009 and 76.0% (225/296) had A/H3N2. The median age of patients was 72 years (IQR 61-82). Males constituted 53.1% (276/520). Median Charlson comorbidity index score was 1 (IQR 0-3). About 70% had physical or radiological evidence of pneumonia upon admission. In-hospital mortality was 58.1% (302/520). On multiple logistic regression analysis, factors positively associated with mortality were age >= 65 years (adjusted odds ratio, aOR = 3.64, 95%CI 2.21-5.99, p < 0.001), malignancy (aOR = 2.53, 95% CI 1.12-5.73; p = 0.026), and hypoalbuminemia (aOR = 2.16, 95%CI 1.26-3.73; p = 0.005), while antiviral therapy (aOR = 0.33, 95%CI 0.17-0.63; p < 0.001) and ventilation (aOR = 0.23, 95% CI 0.13-0.39; p < 0.001) were negatively associated. Conclusions: Patients with severe seasonal influenza infection were characterized by advanced age, hypoalbuminemia and presence of pneumonia on admission. Age >= 65 years, malignancy, and hypoalbuminemia were associated with increased mortality, and antiviral therapy and ventilation with decreased mortality.