Data on a total of 885 adult confirmed H1N1 cases from the CCDC. A confirmed case was defined as a person with influenzalike clinical manifestations and positive laboratory test for H1N1 virus, as assessed with the use of a reverse-transcriptasepolymerase-chain-reaction (RT-PCR) assay. Those patients who were still asymptomatic but positive for the RT-PCR assay were quarantined. The cases included in this study were those reported in the early phase of the epidemic between May 7 th and August 16 th , 2009 to the CCDC by sub-CCDC offices after face to face interviews with the individuals of suspected infection.