Patients hospitalized with laboratory-confirmed influenza during the 2010-2011 influenza season: exploring disease severity by virus type and subtype.

Background. The 2010-2011 influenza season was dominated by influenza A(H3N2) virus, but influenza A(H1N1) pdm09 (pH1N1) and B viruses cocirculated. This provided an opportunity to explore within-season predictors of severity among hospitalized patients, avoiding biases associated with season-to-season differences in strain virulence, population immunity, and healthcare seeking. Methods. Population-based, laboratory-confirmed influenza hospitalization surveillance data were used to examine the association between virus type/subtype and outcomes in children and adults. Multivariable analysis explored virus type/subtype, prompt antiviral treatment, medical conditions, and age as predictors for severity (intensive care unit admission or death). Results. In children, pH1N1 (adjusted odds ratio [aOR], 2.19; 95% confidence interval [CI], 1.11-4.3), chronic metabolic disease (aOR, 5.23; 95% CI, 1.74-15.69), and neuromuscular disorder (aOR, 4.84; 95% CI, 2.02-11.58) were independently associated with severity. In adults, independent predictors were pH1N1 (aOR, 2.21; 95% CI, 1.66-2.94), chronic lung disease (aOR, 1.46, 95% CI, 1.12-1.89), and neuromuscular disorder (aOR, 1.68; 95% CI, 1.11-2.52). Antiviral treatment reduced the odds of severity among adults (aOR, 0.47; 95% CI, .33-.68). Conclusions. During the 2010-2011 season, pH1N1 caused more severe disease than H3N2 or B in hospitalized patients. Underlying medical conditions increased severity despite virus strain. Antiviral treatment reduced severity among adults. Our findings underscore the importance of influenza prevention.