Case–Control Study of Risk Factors for Hospitalization Caused by Pandemic (H1N1) 2009

We conducted a case–control study to identify risk factors for hospitalization from pandemic (H1N1) 2009 virus infection among persons >16 years of age in Sydney, Australia. The study comprised 302 case-patients and 603 controls. In a logistic regression model, after adjusting for age and sex, risk factors for hospitalization were pregnancy (odds ratio [OR] 22.4, 95% confidence interval [CI] 9.2–54.5), immune suppression (OR 5.5, 95% CI 2.8–10.9), pre-existing lung disease (OR 6.6, 95% CI 3.8–11.6), asthma requiring regular preventive medication (OR 4.3, 95% CI 2.7–6.8), heart disease (OR 2.3, 95% CI 1.2–4.1), diabetes (OR 3.8, 95% CI 2.2–6.5), and current smoker (OR 2.0, 95% CI 1.3–3.2) or previously smoked (OR 2.0, 95% CI 1.3–3.0). Although obesity was not independently associated with hospitalization, it was associated with an increased risk of requiring mechanical ventilation. Public health messages should give greater emphasis on the risk for severe disease among pregnant women and smokers.