We used a database that included all cases of pandemic influenza (H1N1-2009) reported to the China Information System for Disease Control and Prevention (CISDCP) from May 9, 2009 , when the first confirmed case in China was reported, to December 31, 2009 (6) . The CISDCP covers all provincial, prefectural, and county centers for disease control and prevention, 95.3% of the provincial, prefectural, and county hospitals (9,084 in total), and 84.0% of township clinics (38,175 in total) across mainland China. After the World Health Organization issued an alert about the novel influenza virus (H1N1-2009), pandemic influenza was classified as a class B notifiable infectious disease on April 30, 2009 , by the Ministry of Health but was managed according to the criteria for class A notifiable infectious diseases. According to the Law for Prevention and Control of Infectious Diseases in China, information regarding each patient, once identified, must be reported to the CCDC within 2 hours through the Web-based CISDCP system. A suspected case was defined as a person with influenza-like symptoms who had had close contact with a confirmed case within the past 7 days, had a history of travel to affected areas within the past 7 days, or tested positive for influenza A virus, excluding other known subtypes of influenza A. A laboratory-confirmed case was defined according to World Health Organization criteria-that is, a person with influenza-like symptoms and laboratory-confirmed pandemic influenza A virus infection by one or more of the following tests: reverse-transcriptase polymerase chain reaction, real-time reverse-transcriptase polymerase chain reaction, viral culture, or a 4-fold rise in specific antibodies to pandemic influenza A virus (7) . Influenzalike symptoms were defined according to World Health Organization criteria: sudden onset of fever greater than 38°C, cough or sore throat, and absence of other diagnoses (8) . All laboratory-confirmed cases from 2009 were included in our database, including information about age, sex, occupation, residence address, work address, onset date and location, hospital admission date and address, and clinical outcome. Furthermore, census information was obtained from the National Bureau of Statistics of China (9) .