Comparative epidemiology of four waves of human infections with avian influenza A(H7N9) in mainland China, 2013-2016

Abstract

BACKGROUND: The novel avian influenza A(H7N9) has caused four epidemic waves of human infection since it was first reported in southeastern China in March 2013. The H7N9 infections mainly zoonotic through exposure to infected poultry or contaminated environment. Closure of live poultry markets was highly effective in prevention of human infections, and many places in China has implemented either permanent or regular temporary closure markets in response to H7N9 outbreaks. We would like to examine the epidemiological characteristics of influenza A(H7N9) cases reported in mainland China in 2013-2016. METHOD: All laboratory-confirmed influenza A(H7N9) cases were reported to the Chinese Center for Disease Control and Prevention in an integrated database in which demographic, epidemiological and basic clinical information was included. We analyzed cases by age, sex, residence, type of exposure, and other key epidemiologic variables. Epidemiological time-to-event distributions including infection to symptom onset, onset to admission, onset to laboratory confirmation, hospital admission to death, and admission to discharge were examined using kernel density methods. RESULTS: As of 3 March 2016, 731 laboratory-confirmed human cases of influenza A(H7N9) have reported mostly in the south and east of mainland China. Most cases were male, elderly and urban residents. Almost all laboratory-confirmed H7N9 cases were hospitalized. Around 60% of H7N9 cases reported having underlying medical conditions. Recent exposure to poultry was reported by 80% of cases among which over 70% reported visiting a live poultry market during within 2 weeks before symptom onset. The mean time from illness onset to hospitalization was approximately 2 days while laboratory-confirmation was largely obtained within 10 days after onset The mean duration of hospitalization was relatively short whereas with a substantial variability in the duration of hospitalization among all cases. CONCLUSION: The avian influenza A(H7N9) virus has continued to pose threat to public health in mainland China since its first emergence. Elderly, male and urban residents who had underlying medical conditions had a high risk of severe illness associated with H7N9 infection. Effective interventions to reduce human exposure to infected live poultry should be considered to further reduce the disease burden in the future.