Risk factors for severe outcomes among members of the United States military hospitalized with pneumonia and influenza, 2000-2012.

backgroundThe progression from hospitalization for a respiratory infection to requiring substantial supportive therapy is a key stage of the influenza severity pyramid. Respiratory infections are responsible for 300,000-400,000 medical encounters each year among US military personnel, some of which progress to severe acute respiratory infections.

methodsWe obtained data on 11,086 hospitalizations for pneumonia and influenza (P&I) among non-recruit US military service members during the period of 1 January 2000 through 31 December 2012. From these, we identified 512 P&I hospitalizations that progressed to severe episodes using standard case definitions. We evaluated the effect of demographic and occupational characteristics, co-morbid conditions, and history of influenza vaccination on the risk of a hospitalized P&I case becoming a severe case. We also evaluated the risk of a severe outcome and the length of time since influenza vaccination (within 180, 60, and 30 days).

resultsThe median age of subjects at the time of the P&I episode was 32 years (range, 28-40) and subjects were predominantly male (89.5%). In a univariate analysis, demographic risk factors for a severe episode included service in the US Air Force (RR=1.6 relative to US Army, 95%CI 1.3-2.1), US Coast Guard (RR=2.1, 1.2-3.7) or US Navy (RR=1.4, 1.1-1.8). Being born in the US and recent influenza vaccination (within 180 days of episode) were protective against developing severe disease. Among co-morbid conditions, univariate risk factors for severe disease included chronic renal or liver disease (RR=4.98, 95%CI 4.1-6.1), diseases of the circulatory system (RR=3.1, 95%CI 2.6-3.7), diabetes mellitus (RR=2.3, 95%CI 1.5-3.6), obesity (RR=1.6, 95%CI 1.2-2.1), cancer (RR=1.6, 95%CI 1.3-2.0), and chronic obstructive pulmonary disease (RR=1.4, 95%CI 1.1-1.7). Although many of the risk factors found to be significant in univariate analysis were no longer significant under a multivariate analysis, receipt of any influenza vaccine within 180 days of episode remained protective (RR=0.81, 95%CI 0.67-0.99), while serving in the US Coast Guard (RR=1.9, 95%CI 1.1-3.4) or US Air Force (RR=1. 5, 95%CI 1.2-2.0), presence of renal or liver disease (RR=3.6, 95%CI 2.9-4.6), and diseases of the circulatory system (RR=2.2, 95%CI 1.8-2.8), remained significantly associated with a higher risk of developing severe disease.

conclusionsIn a large cohort, after adjusting for many possible risk factors, influenza vaccination was protective against severe episodes among P&I hospitalizations. The service-specific (US Coast Guard or US Air Force) increased risk may represent some differences in data (e.g., coding or reporting practices) as opposed to genuine differences in physiological outcome. Our findings suggest that renal and liver disease as well as diseases of the circulatory system may contribute to influenza severity in this population independently of age and other potential comorbidities. These findings provide additional evidence for the prioritization of specific risk groups within the US military for influenza vaccination.