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To investigate the epidemiological characteristics of human infections with avian influenza A (H7N9) in China and to provide scientific evidence for the adjustment of preventive strategy and control measures.Demographic and epidemiologic information on human cases were collected from both reported data of field epidemiological investigation and the reporting system for infectious diseases.A total of 433 cases including 163 deaths were reported in mainland China before June 4, 2014. Two obvious epidemic peaks were noticed, in March to April, 2013 and January to February, 2014. Confirmed cases emerged in 14 areas of China. Five provinces, including Zhejiang, Guangdong, Jiangsu, Shanghai, and Hunan, reported about 85% of the total cases. Median age of the confirmed cases was 58 years (range, 1-91), with 70% as males. Of the 418 cases with available data, 87% had ever exposed to live poultry or contaminated environments. 14 clusters were identified but human to human transmission could not be ruled out in 9 clusters.Human infections with avian influenza A (H7N9) virus showed the characteristics of obvious seasonal distribution, with certain regional clusters. The majority of confirmed cases were among the elderly, with more males seen than the females. Data showed that main source of infection was live poultry and the live poultry market had played a significant role in the transmission of the virus.

In mainland China, most avian influenza A(H7N9) cases in the spring of 2013 were reported through the pneumonia of unknown etiology (PUE) surveillance system. To understand the role of possible underreporting and surveillance bias in assessing the epidemiology of subtype H7N9 cases and the effect of live-poultry market closures, we examined all PUE cases reported from 2004 through May 3, 2013. Historically, the PUE system was underused, reporting was inconsistent, and PUE reporting was biased toward A(H7N9)-affected provinces, with sparse data from unaffected provinces; however, we found no evidence that the older ages of persons with A(H7N9) resulted from surveillance bias. The absolute number and the proportion of PUE cases confirmed to be A(H7N9) declined after live-poultry market closures (p

BACKGROUND: The first identified cases of avian influenza A(H7N9) virus infection in humans occurred in China during February and March 2013. We analyzed data obtained from field investigations to describe the epidemiologic characteristics of H7N9 cases in China identified as of December 1, 2013. METHODS: Field investigations were conducted for each confirmed case of H7N9 virus infection. A patient was considered to have a confirmed case if the presence of the H7N9 virus was verified by means of real-time reverse-transcriptase–polymerase-chain-reaction assay (RT-PCR), viral isolation, or serologic testing. Information on demographic characteristics, exposure history, and illness timelines was obtained from patients with confirmed cases. Close contacts were monitored for 7 days for symptoms of illness. Throat swabs were obtained from contacts in whom symptoms developed and were tested for the presence of the H7N9 virus by means of real-time RT-PCR. RESULTS: Among 139 persons with confirmed H7N9 virus infection, the median age was 61 years (range, 2 to 91), 71% were male, and 73% were urban residents. Confirmed cases occurred in 12 areas of China. Nine persons were poultry workers, and of131 persons with available data, 82% had a history of exposure to live animals, including chickens (82%). A total of 137 persons (99%) were hospitalized, 125 (90%) had pneumonia or respiratory failure, and 65 of 103 with available data (63%) were admitted to an intensive care unit. A total of 47 persons (34%) died in the hospital after a median duration of illness of 21 days, 88 were discharged from the hospital, and 2 remain hospitalized in critical condition; 2 patients were not admitted to a hospital. In four family clusters, human-to-human transmission of H7N9 virus could not be ruled out. Excluding secondary cases in clusters, 2675 close contacts of case patients completed the monitoring period; respiratory symptoms developed in 28 of them (1%); all tested negative for H7N9 virus. CONCLUSIONS: Most persons with confirmed H7N9 virus infection had severe lower respiratory tract illness, were epidemiologically unrelated, and had a history of recent exposure to poultry. However, limited, nonsustained human-to-human H7N9 virus transmission could not be ruled out in four families.