Assessment of human-to-human transmissibility of avian influenza A(H7N9) virus across five waves by analyzing clusters of case-patients in mainland China, 2013-2017.
Wang X., Wu P., Pei Y., Tsang TK., Gu D., Wang W., Zhang J., Horby PW., Uyeki TM., Cowling BJ., Yu H.
Background: The 2016-17 epidemic of human infections with avian influenza A(H7N9) virus was alarming due to the surge in reported cases across a wide geographic area and the emergence of highly pathogenic A(H7N9) viruses. Our study aimed to assess whether human-to-human transmission risk of A(H7N9) virus has changed across the five waves since 2013. Methods: Data on human cases and clusters of A(H7N9) virus infection were collected from WHO, national and provincial reports, informal online sources and published literature. We compared the epidemiological characteristics and clinical severity of sporadic and cluster cases, estimated the relative risk (RR) of infection in blood-relatives and non-blood relatives, and estimated the bounds on the effective reproductive number (Re) across waves from 2013 through September 2017. Results: 40 human clusters of A(H7N9) virus infection were identified, with a median cluster size of 2 (range 2-3). The overall RR of infection in blood-relatives versus non-blood relatives was 1.65 (95% CI 0.88, 3.09), and was not significantly different across waves (χ2=2.66, P=0.617). The upper and lower limits of Re for A(H7N9) virus were 0.12 (95% CI 0.10, 0.14) and 0.03 (95% CI 0, 0.14), respectively. Re estimates were not significantly different across waves (χ2=1.52, P=0.822 for upper limit; χ2=6.58, P=0.160 for lower limit). Conclusions: The small cluster size and low Re suggest that human-to-human transmissibility of A(H7N9) virus has not changed over time and remains limited to date. Continuous assessment of A(H7N9) virus infections and human case clusters is of crucial importance for public health.