Human infection with avian influenza A H7N9 virus: an assessment of clinical severity

Abstract

Background Characterizing the severity profile of human infections with influenza viruses of animal origin is a part of pandemic risk assessment, and an important part of the assessment of disease epidemiology. Our objective was to assess the clinical severity of human infections with the avian influenza A(H7N9) virus that has recently emerged in China. Methods Among laboratory-confirmed cases of A(H7N9) who were hospitalised, we estimated the risk of fatality, mechanical ventilation, and admission to the intensive care unit based on censored data during the currently ongoing outbreak. We also used information on laboratory-confirmed cases detected through sentinel influenza-like illness (ILI) surveillance to estimate the number of symptomatic A(H7N9) virus infections to date and the symptomatic case fatality risk. Findings Among 123 hospitalised cases, 37 cases had died and 69 had recovered by May 28, 2013. Hospitalised cases had high risks of mortality (36%; 95% confidence interval (CI): 26%–45%), mechanical ventilation or mortality (69%; 95% CI: 60%–77%), and ICU admission or mechanical ventilation or mortality (83%; 95% CI: 76%–90%), and the risk of these severe outcomes increased with age. Depending on assumptions about the coverage of the sentinel ILI network and health-care seeking behavior for cases of ILI associated with A(H7N9) virus infection, we estimated that the symptomatic case fatality risk could be between 160 and 2,800 per 100,000 symptomatic cases. Interpretation We estimated that the severity of A(H7N9) is somewhat lower than A(H5N1) but higher than seasonal influenza viruses and influenza A(H1N1)pdm09 virus. The estimated risks of fatality among hospitalised cases and symptomatic cases are measures of severity that should not be affected by shifts over time in the probability of laboratory-confirmation of mild cases and should inform risk assessment. Funding Ministry of Science and Technology, China; Research Fund for the Control of Infectious Disease and University Grants Committee, Hong Kong Special Administrative Region, China; and the US National Institutes of Health.