Background: Transmission of the novel avian influenza A H7N9 virus seems to be predominantly between poultry and people. In the major Chinese cities of Shanghai, Hangzhou, Huzhou, and Nanjing--where most human cases of infection have occurred--live poultry markets (LPMs) were closed in April, 2013, soon after the initial outbreak, as a precautionary public health measure. Our objective was to quantify the effect of LPM closure in these cities on poultry-to-person transmission of avian influenza A H7N9 virus.

Methods: We obtained information about every laboratory-confirmed human case of avian influenza A H7N9 virus infection reported in the four cities by June 7, 2013, from a database built by the Chinese Center for Disease Control and Prevention. We used data for age, sex, location, residence type (rural or urban area), and dates of illness onset. We obtained information about LPMs from official sources. We constructed a statistical model to explain the patterns in incidence of cases reported in each city on the basis of the assumption of a constant force of infection before LPM closure, and a different constant force of infection after closure. We fitted the model with Markov chain Monte Carlo methods.

Findings: 85 human cases of avian influenza A H7N9 virus infection were reported in Shanghai, Hangzhou, Huzhou, and Nanjing by June 7, 2013, of which 60 were included in our main analysis. Closure of LPMs reduced the mean daily number of infections by 99% (95% credibility interval 93-100%) in Shanghai, by 99% (92-100%) in Hangzhou, by 97% (68-100%) in Huzhou, and by 97% (81-100%) in Nanjing. Because LPMs were the predominant source of exposure to avian influenza A H7N9 virus for confirmed cases in these cities, we estimated that the mean incubation period was 3·3 days (1·4-5·7).

Interpretation: LPM closures were effective in the control of human risk of avian influenza A H7N9 virus infection in the spring of 2013. In the short term, LPM closure should be rapidly implemented in areas where the virus is identified in live poultry or people. In the long term, evidence-based discussions and deliberations about the role of market rest days and central slaughtering of all live poultry should be renewed.

Funding: Ministry of Science and Technology, China; Research Fund for the Control of Infectious Disease; Hong Kong University Grants Committee; China-US Collaborative Program on Emerging and Re-emerging Infectious Diseases; Harvard Center for Communicable Disease Dynamics; and the US National Institutes of Health.