Pneumonia risk factors and clinical features of hospitalized patients older than 15 years with pandemic influenza A (H1N1) in South Korea: a multicenter study

While most patients with pandemic influenza A (H1N1) virus had a self-limited disease, some patients had severe pneumonia requiring hospitalization. We conducted a retrospective review to characterize demographically the patients older than 15 years with H1N1 infection requiring hospitalization and assess the risk factors for pneumonia development. The most common underlying medical condition was a respiratory disorder such as asthma or chronic bronchitis (11.4%). The symptoms most common at presentation included fever and cough (91.9% and 71.2%, respectively). Of the 210 patients studied, 55 (26.2%) had viral pneumonia. The independent risk factors for pneumonia were asthma [odds ratio (OR) 4.006], male gender (OR 3.507), and age ≥50 years (OR 2.653). There were insufficient numbers of pregnant, obese, and diabetic patients to allow statistical analyses. Early treatment with antiviral agents was associated with reduced risk of pneumonia (OR 0.822). Our results suggest that asthma, old age, and male gender may be related to pneumonia and that early antiviral treatment may reduce pneumonia risk.