Obesity as a risk factor for severe influenza-like illness

Background Obesity was recognized as in independent risk factor for influenza during the 2009 H1N1 influenza pandemic. Objectives We evaluated the association between body mass index (BMI) and influenza-like illness (ILI) during two non-pandemic influenza seasons (2003-2004 and 2004-2005) and during the spring and fall waves of the 2009 H1N1 pandemic. Methods Adults with severe (inpatient) and mild (outpatient) ILI were compared to those without ILI using a case-cohort design. The study was nested among those insured by a single health insurance company, receiving care from a large multispecialty practice. Data were collected from insurance claims and the electronic health record. The primary exposure was obesity (BMI300kg/m(2)). Results Across three seasons, the crude and adjusted ORs for obesity and severe ILI were 165 (95% CI 131, 208) and 123 (95% CI 097, 157), respectively. An association was observed for those aged 20-59years (adjusted OR 192, 95% CI 126, 290), but not for those 60 and older (adjusted OR 108, 95% CI 080, 146). The adjusted ORs for obesity and severe ILI in 2003-2004, 2004-2005, and during H1N1 were 114 (95% CI 080, 164), 124 (95% CI 086, 179), and 176 (95% CI 091, 342), respectively. Among those with a Charlson Comorbidity Index score of zero, the adjusted ORs for 2003-2004, 2004-2005, and H1N1 were 160 (95% CI 093, 276), 143 (95% CI 080, 256), and 190 (95% CI 068, 527), respectively. Conclusions Our results suggest a small to moderate association between obesity and hospitalized ILI among adults.