Mild form of 2009 H1N1 influenza infection detected by active surveillance: Implications for infection control

BackgroundScreening patients with suspected influenza is a key step for infection control within communities and institutions. By analyzing the clinical characteristics of mild 2009 H1N1 influenza cases detected by active surveillance, we assessed the utility of the commonly used influenza case definition.MethodsWe retrospectively reviewed medical records of 44 patients who were hospitalized and quarantined and who tested positive for the 2009 H1N1 virus using real-time reverse-transcriptase polymerase chain reaction between May 29 and July 28, 2009.ResultsPatient median age was 17 years (range, 8-79 years), and 37 patients were male (84%). Common symptoms included cough (34/44; 77.3%), subjective fever (23/44; 52.3%), rhinorrhea or nasal congestion (22/44; 50%), sore throat (19/44; 43.2%), and diarrhea (7/44; 15.9%). All patients were treated with oseltamivir after the onset of initial symptoms (mean, 2.6 days). Common laboratory test results included leucopenia (23/44; 52.3%) and mildly elevated C-reactive protein (26/44; 59.1%).ConclusionThere were many mild afebrile cases of the 2009 pandemic H1N1 influenza. Cough, mild leukopenia, and mildly elevated C-reactive protein were relatively common clinical manifestations. Thus, case-based surveillance for the index cluster of 2009 pandemic influenza is not an effective method for infection control in communities or hospital settings.