Prevalence and clinical features of pneumonia in patients with laboratory-confirmed pandemic influenza A H1N1 2009 infection in South Korea

During the early stages of the pandemic influenza A H1N1 2009 (pH1N1) outbreak in South Korea, the government recommended antiviral therapy with laboratory confirmation in specialized hospitals. Hence we had a unique opportunity to test all patients suspected to have pH1N1, including those who initially presented with a mild illness, such as those who were not in at-risk groups and who had an uncomplicated illness. We therefore evaluated the proportion and clinical features of pneumonia in patients with laboratory-confirmed pH1N1. Of the 3253 patients who visited Asan Medical Center, Seoul, South Korea, between 24 August and 19 October 2009 for a suspected pH1N1 infection (temperature >= 37.5 degrees C and at least 1 of the following symptoms: sore throat, cough, rhinorrhoea and nasal congestion), 553 (17%) were positive for pH1N1 by real-time reverse-transcriptase polymerase chain reaction. Chest radiographs were performed in 96 (17%) of the 553 patients. Of the 553 patients, 30 (5%; 95% confidence interval (CI) 4-8%) had pneumonia, including 26 (86%) with viral pneumonia and 4 (14%) with bacterial pneumonia. So, the proportion of pneumonia in patients with laboratory-confirmed pH1N1 was at least 5%. Multivariate analysis indicated that dyspnoea (odds ratio (OR) 57.8, p < 0.001), wheezing (OR 19.3, p = 0.02), vomiting (OR 18.5, p < 0.001) and diarrhoea (OR 11.0, p = 0.001) were independently associated with pneumonia. Antiviral therapy at > 48 h after the onset of symptoms (OR 2.1, p = 0.09) tended to be more common in patients with pneumonia than in those without pneumonia.