In 2009, under the guidance of the Beijing Center for Disease Prevention and Control (Beijing CDC), a network of 55 collaborating laboratories was established to perform reverse transcription PCR testing to confi rm cases of pandemic (H1N1) 2009 (15) . The confi rmed cases included symptomatic and asymptomatic cases, and these cases were detected mainly by border entry screening, ILI screening in hospitals, health follow-up of travelers from overseas, and quarantine and testing of close contacts. Once confi rmed, index case-patients were immediately quarantined in designated hospitals to receive treatment while in isolation. All the confi rmed cases were required by law to be reported to Beijing and local CDCs. From May through October 2009, a detailed epidemiologic investigation was conducted for each confi rmed case of pandemic (H1N1) 2009 (including symptomatic and asymptomatic cases) by Beijing and local CDCs within 6 hours after confi rmation of infection. Patients with confi rmed cases were interviewed about demographic characteristics, course of illness, travel and contact history, and information about close contacts. Patients with confi rmed cases were categorized as having imported cases (travelers) and locally acquired cases (no travel history) on the basis of where the infection was acquired.