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 characterize the epidemiologic characteristics of H7N9 cases in China as of April 17, 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 (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 testing by means of real-time RT-PCR.

Results

Among 82 persons with confirmed H7N9 virus infection, the median age was 63 years (range, 2 to 89), 73% were male, and 84% were urban residents. Confirmed cases occurred in six areas of China. Of 77 persons with available data, 4 were poultry workers, and 77% had a history of exposure to live animals, including chickens (76%). A total of 17 persons (21%) died after a median duration of illness of 11 days, 60 remain critically ill, and 4 with clinically mild cases were discharged from the hospital; 1 pediatric patient was not admitted to the hospital. In two family clusters, human-to-human transmission of H7N9 virus could not be ruled out. A total of 1251 of the 1689 close contacts of case patients completed the monitoring period; respiratory symptoms developed in 19 of them (1.5%), all of whom tested negative for the H7N9 virus.

Conclusions

Most persons with confirmed H7N9 virus infection were critically ill and epidemiologically unrelated. Laboratory-confirmed human-to-human H7N9 virus transmission was not documented among close contacts, but such transmission could not be ruled out in two families.