Objective
Influenza is the most common vaccine-preventable disease in the United States; however, little is known about the burden of critical illness due to influenza virus infection. Our primary objective was to estimate the proportion of all critical illness hospitalizations that are attributable to seasonal influenza.
Design
Retrospective cohort study
Setting
Arizona, California, and Washington from January 2003 through March 2009
Patients
All adults hospitalized with critical illness, defined by ICD-9-CM diagnosis and procedure codes for acute respiratory failure, severe sepsis, or in-hospital death
Measurements
We combined the complete hospitalization discharge databases for three US states, regional influenza virus surveillance, and state census data. Using negative binomial regression models, we estimated the incidence rates of adult influenza-associated critical illness hospitalizations and compared them to all-cause event rates. We also compared modeled outcomes to ICD-9-CM-coded influenza hospitalizations to assess potential under-recognition of severe influenza disease.
Main Results
During the study period, we estimated that 26,760 influenza-associated critical illness hospitalizations (95% CI: 14,541, 47,464) occurred. The population-based incidence estimate for influenza-associated critical illness was 12.0 per 100,000 person-years (95% CI: 6.6, 21.6), or 1.3% (95%CI:0.7%, 2.3%) of all critical illness hospitalizations. During the influenza season, 3.4% (95%CI:1.9%, 5.8%) of all critical illness hospitalizations were attributable to influenza. There were only 2,612 critical illness hospitalizations with ICD-9-CM-coded influenza diagnoses, suggesting influenza is either undiagnosed or under-coded in a substantial proportion of critical illness.
Conclusions
Extrapolating our data to the 2010 US population, we estimate about 28,000 adults are hospitalized for influenza-associated critical illness annually. Influenza in many of these critically ill patients may be undiagnosed. Critical care physicians should have a high index of suspicion for influenza in the ICU, particularly when influenza is known to be circulating in their communities.