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 to March 2009.
Patients: All adults hospitalized with critical illness, defined by International Classification of Diseases, 9th Edition, Clinical Modification diagnosis and procedure codes for acute respiratory failure, severe sepsis, or in-hospital death.
Measurements and main results: We combined the complete hospitalization discharge databases for three U.S. 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 with all-cause event rates. We also compared modeled outcomes to International Classification of Diseases, 9th Edition, Clinical Modification-coded influenza hospitalizations to assess potential underrecognition of severe influenza disease. 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% of all critical illness hospitalizations (95% CI, 0.7%, 2.3%). During the influenza season, 3.4% of all critical illness hospitalizations (95% CI, 1.9%, 5.8%) were attributable to influenza. There were only 2,612 critical illness hospitalizations with International Classification of Diseases, 9th Edition, Clinical Modification-coded influenza diagnoses, suggesting influenza is either undiagnosed or undercoded in a substantial proportion of critical illness.
Conclusions: Extrapolating our data to the 2010 U.S. population, we estimate that 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.