The (H1N1) 2009 pandemic influenza pneumonia among adult patients in Japan.

The recent H1N1 influenza pandemic involved several cases of influenza pneumonia. Although influenza pneumonia may have occurred more frequently in fatal cases, the clinical characteristics of influenza pneumonia in Japan remain unclear. We conducted a retrospective cohort study of adult patients with influenza pneumonia, for which questionnaires were sent to respiratory physicians working in 2,491 hospitals across Japan. Questionnaires were returned by 994 physicians (39.9%), providing data on 346 influenza pneumonia cases. The case-fatality ratio was 8.2% (27 cases). Pure influenza viral pneumonia was observed in 94 cases, which were most frequently young adults with chief complaints of non-productive cough and dyspnea. Radiological imaging showed diffuse ground-glass opacity. Corticosteroid therapy and mechanical ventilation were more frequently used for pure influenza viral pneumonia, but these modalities were not correlated with poor treatment outcomes. Anti-influenza antiviral drugs were prescribed in 335 cases (96.8%). In the recent pandemic in Japan, several pure influenza viral pneumonia cases were observed, but we found no variances in mortality between the different types of pneumonia. Almost all cases were treated with anti-influenza antiviral drugs, which may have contributed to its relatively low mortality rate.