Clinical features of the 2009 swine-origin influenza A (H1N1) outbreak in Japan

To clarify the clinical symptoms of the influenza A virus during the 2009 pandemic influenza outbreak, we describe the clinical features of outpatients diagnosed with type A influenza by use of the rapid influenza diagnostic test (RIDT) from September to December 2009. Questionnaires were used to collect prospective data on 1,122 cases with influenza-like illness at our medical institutions. The independent predictors of influenza A virus were identified on the basis of demographic features and the clinical symptoms of the patients who tested positive for influenza A virus in the RIDT test. Of the 1,122 cases tested, 389 (34.7%) were positive for the influenza A virus. The median age of the influenza-positive patients was 14, and 58.9% of the patients were male. The symptoms fever, cough, rhinorrhea, and headache were statistically dominant. A history of recent contact with persons suffering from influenza or influenza-like illness at home, school, or in the workplace was significantly more common in the positive group than in the negative group. Pneumonia was observed in 2 (0.5%) of the positive patients, but the symptoms were only severe enough to require hospitalization in 1 of the 2. No deaths were observed among the 389 RIDT-positive patients. Although the spread of influenza A virus was both rapid and extensive, mainly among children under the age of 18, it seemed to be mild. Appropriate interpretation of the RIDT on the basis of recent clinical information, and early treatment with antiviral drugs might help to prevent severe illness from influenza pandemics in the future.