Underlying Medical Conditions and Hospitalization for Pandemic (H1N1) 2009, Japan

To the Editor: Early epidemio-logic reports suggested that infection with pandemic (H1N1) 2009 virus most commonly occurred in teenagers and young adults (1). Although this infection appears to have a mild clinical course and the mortality rate has been relatively low in Japan (1.6 deaths/1 million population) (2), the reported number of patients with severe (requiring intubation or admission to an intensive care unit) cases of this disease has been increasing (2). The difference in severity of pandemic (H1N1) 2009 infection may be attributed to differences in underlying medical conditions or to age-related differences in susceptibility. To explore these differences, we investigated the incidence of laboratory-confi rmed cases of pandemic (H1N1) 2009 virus infections resulting in hospitalization in Japan and the patients' age-specifi c risks for hospitalization associated with underlying medical conditions. During the outbreak in Japan, hospitals , local public health centers, and local governments were required to report hospitalizations associated with pandemic (H1N1) 2009 virus to the Ministry of Health, Labour and Welfare in Japan. Infection was confi rmed by PCR at local public health centers. The data for hospitalized patients were integrated through a nationwide surveillance system, the interim National Epidemiologic Surveillance of Infectious Diseases. We collected data on patients who were hospitalized in Japan for pandem-The number of new cases (incidence) increased in the middle of August and peaked at the end of November (2). We excluded from analysis those patients who had been hospitalized for the purpose of infection containment. Medical conditions were defi ned as diseases that are risk factors for severe illness associated with seasonal infl uenza: chronic respiratory diseases, chronic cardiovas-cular diseases, chronic renal diseases, chronic liver diseases, neurologic diseases , hematologic diseases, diabetes, and immunosuppression caused by treatments or illnesses (including malignant neoplasm) and chronic childhood diseases (3,4). Data on the prevalence of the diseases among the population were obtained from the 2005 National Patient Survey and from population estimates obtained by the Ministry of Internal Affairs and Communications in May 2009. During the surveillance period, 12,702 laboratory-confi rmed cases for which the patients required hospitalization were reported; 110 of these patients died (Table). Hospitalization incidence was 10.0 admissions/100,000 persons (overall incidence of hospitalization in the general population in 2005 was 1,462.8 admissions/100,000 persons). Median age of hospitalized patients was 7 years (interquartile range 5–11 years). Of the 10,721 patients for whom type of care was known, 680 (6.4%) were admitted …