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Abstract and Introduction
Abstract
Background/Objective This case–control study was carried out to estimate risk factors associated with hospitalizations and severe outcomes [intensive care unit (ICU) admission or death] among patients with illness because of laboratory-confirmed 2009 pandemic A/H1N1 virus (pH1N1) during the first wave of pH1N1 activity in the province of Quebec, Canada.
Patients/Methods We collected epidemiologic information by phone using a standardized questionnaire from patients with laboratory-confirmed pH1N1 illness during the first spring/summer pandemic wave in Quebec, Canada. Risk factors associated with hospitalization were assessed by comparing hospitalized to community cases and for ICU admission or death through comparison with hospitalized cases.
Results Cases (321 hospitalized patients including 47 ICU admissions and 15 deaths) were compared to controls (395 non-hospitalized patients) by using multivariable logistic regression adjusted for gender, age, education, being a health care worker, smoking, seasonal influenza vaccination, delay to consultation, antiviral use before admission, pregnancy, underlying medical conditions, and obesity. Age <5 years, underlying medical conditions (neuromuscular, cardiac, pulmonary, and renal conditions, diabetes, asthma, and other), and delayed consultation were associated with hospitalization. The strongest association with hospitalization was observed for neuromuscular disorders. Antiviral medication before hospital admission protected against severe disease. Association of obesity with hospitalization was not significant after adjustment in multivariable analysis. Among hospitalized patients, age ≥60 years and immune suppression were associated with death.
Conclusions Previously identified risk factors for seasonal influenza were also associated with increased risk of severe pH1N1 outcomes. The independent role of obesity needs to be further defined.
Introduction
As with seasonal influenza, the vast majority of patients with illness caused by the 2009 pandemic A/H1N1 virus (pH1N1) experienced an uncomplicated course to recovery. Similar to previous pandemics, the median age of patients presenting with pH1N1 was younger than that of patients with seasonal influenza. Surveillance reports, outbreak investigations, descriptive case series, and a few case–control studies have attempted to characterize the profile of patients experiencing more severe outcomes. The estimated rate of hospitalization among symptomatic patients with pH1N1 varied by age, ranging from 0·53% in children 5–17 years of age, 2·19% in children <5 years of age, and up to 4·0% in adults. Among hospitalized cases, between 6% and 31% were admitted to the intensive care unit (ICU) and up to 5% died. Secondary bacterial infections have been confirmed in 20–43% of severe cases.
Most patients with severe pH1N1 illness had underlying medical conditions known also to increase the risk for complications from seasonal influenza. Obesity, which was not previously recognized as a risk factor for severe influenza, has been observed in a substantial proportion of patients with severe pH1N1 illness. Two recent studies comparing hospitalizations and deaths from pH1N1 with the source population suggested that obesity may be independently associated with severe outcomes because of pH1N1.
We conducted a case–control study to assess risk factors associated with hospitalization and severe outcomes (ICU admission or death) among patients with confirmed pH1N1 illness during the first pandemic wave in the province of Quebec, Canada.
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