A Novel Risk Factor for a Novel Virus: Obesity and 2009 Pandemic Influenza A (H1N1)

Background. Many critically ill patients with 2009 pandemic influenza A (H1N1) (2009 H1N1) infection were noted to be obese, but whether obesity, rather than its associated co-morbidities, is an independent risk factor for severe infection is unknown. Methods. Using public health surveillance data, we analyzed demographic and clinical characteristics of California residents hospitalized with 2009 H1N1 infection to assess whether obesity (body mass index [BMI] ≥30) and extreme obesity (BMI ≥40) were an independent risk factor for death among case patients ≥ 20 years old. Results. During the period 20 April–11 August 2009, 534 adult case patients with 2009 H1N1 infection for whom BMI information was available were observed. Two hundred twenty-eight patients (43%) were ≥50 years of age, and 378 (72%) had influenza-related high-risk conditions recognized by the Advisory Committee on Immunization Practices as risk factors for severe influenza. Two hundred and seventy-four (51%) had BMI ≥30, which is 2.2 times the prevalence of obesity among California adults (23%) and 1.5 times the prevalence among the general population of the United States (33%). Of the 92 case patients who died (17%), 56 (61%) had BMI ≥30 and 28 (30%) had BMI ≥40. In multivariate analysis, BMI ≥40 (odds ratio [OR], 2.8; 95% confidence interval [CI], 1.4–5.9) and BMI ≥45 (OR, 4.2; 95% CI, 1.9–9.4), age ≥50 years (OR, 2.1; 95% CI, 1.2–3.7), miscellaneous immunosuppressive conditions (OR, 3.9; 95% CI, 1.6–9.5), and asthma (OR, 0.5; 95% CI, 0.3–0.9) were associated with death. Conclusion. Half of Californians ≥20 years of age hospitalized with 2009 H1N1 infection were obese. Extreme obesity was associated with increased odds of death. Obese adults with 2009 H1N1 infection should be treated promptly and considered in prioritization of vaccine and antiviral medications during shortages.