Background.?The 2010-11 influenza season was dominated by influenza A(H3N2) virus, but influenza A(H1N1)pdm09 (pH1N1) and B viruses co-circulated. 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 (ICU admission or death).Results.?In children, pH1N1 (adjusted odds ratio [aOR] 2.19; 95% 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 0.33-0.68).Conclusions.?During 2010-11 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.