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Volume 17, Number 8–August 2011
Case–Control Study of Risk Factors for Hospitalization Caused by Pandemic (H1N1) 2009
Author affiliation: New South Wales Department of Health, Sydney, New South Wales, Australia
Suggested citation for this article
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.
The emergence of pandemic (H1N1) 2009 virus (1,2) was associated with a large increase in the number of persons requiring hospitalization for severe influenza disease in many parts of the world (3–5). In response, in an effort to reduce the impact of the pandemic on their communities and health services, public health agencies developed recommendations for persons at increased risk for disease to seek early treatment. However, these recommendations were based on studies of seasonal influenza (6–10) and descriptive case reports (11–20).
The first cases of pandemic (H1N1) 2009 infection from New South Wales (NSW) were reported in May 2009. In NSW, laboratories were required to notify all pandemic (H1N1) 2009 diagnoses to the NSW Department of Health under Public Health Act 1991 (21). Australian public health management protocols recommended laboratory testing for all persons with influenza-like illness (fever and cough or sore throat) admitted to a hospital (22). Public health follow-up was required to ascertain hospitalization status for all notified cases at the time of diagnosis; this information was collated on a statewide database. Within Sydney, the capital city of NSW (population 4.4 million), there are 4 Area Health Services (AHSs) responsible for the provision of local public health and clinical services.
By the end of June 2009 (before a vaccine was available), community transmission was widespread in Australia, and public health efforts were focused on protecting those at greatest risk for severe disease. The groups considered most vulnerable were pregnant women; indigenous people; very obese persons; persons with pre-existing chronic medical conditions, including lung, heart, and kidney disease; and persons with blood, metabolic or neurologic disorders, immunosuppressive conditions, or asthma (23). Persons in these risk groups and those with severe disease were urged to seek medical attention early if influenza like symptoms appeared. Doctors were provided with free antiviral medication for patients who were seen within 48 hours of symptom onset. A vaccine became publicly available for distribution in September 2009. To help reach populations who would most benefit from prevention and early intervention, we sought to identify independent risk factors for moderate to severe disease from pandemic (H1N1) 2009 infection among adults and to describe the characteristics of those who sought early medical treatment to determine the effectiveness of public health messages.
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Suggested Citation for this Article
Ward KA, Spokes PJ, McAnulty JM. Case–control study of risk factors hospitalization caused by pandemic (H1N1) 2009. Emerg Infect Dis [serial on the Internet]. 2011 Aug [date cited]. http://www.cdc.gov/EID/content/17/8/100842.htm
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Paula J. Spokes, New South Wales Department of Health, 73 Miller St, Sydney, NSW 2060, Australia; email: email@example.com