Influenza H 5 / H 7 Virus Vaccination in Poultry and Reduction of Zoonotic Infections

H infections with avian influenza A(H7N9) virus have been documented in China since 2013 (1). Among 1,220 confirmed H7N9 case-patients during 2013– 2017, a total of 73% reported poultry exposure, and 57% had visited live poultry markets (LPMs) before symptom onset (2). Because of the lack of apparent clinical signs in poultry infected with low pathogenicity H7N9 influenza virus, it has been challenging to rapidly identify and remove infected poultry at the LPMs or farms and to justify implementation of mandatory vaccination of poultry against this virus. Interventions such as market closure during human epidemics have temporarily reduced human exposure to live poultry and decreased zoonotic infection risk (3). However, the effect was not sustainable because H7N9 viruses continue to circulate within the LPM supply chain, leading to recurrent waves of human infections in winter months (4). During winter 2016–17, the H7N9 virus evolved into highly pathogenic avian influenza (HPAI) virus by acquiring a polybasic amino acid motif at the hemagglutinin cleavage site, rendering the virus capable of disseminating systematically and causing high mortality rates among chickens (5,6). In response to the emergence of HPAI H7N9 virus, the government of China amended the mandatory vaccination regimen for avian influenza in summer 2017. Specifically, a newly developed bivalent H5 (Re-8, based on clade 2.3.4.4 H5N1 virus A/chicken/Guizhou/4/2013) and H7 (Re-1, based on H7N9 virus A/pigeon/Shanghai/S1069/2013) vaccine replaced the previous bivalent H5 vaccine that targeted H5 clades 2.3.4.4 (Re-8) and 2.3.2.1 (Re-6, based on H5N1 virus A/duck/Guangdong/S1322/2010). The new bivalent H5/H7 vaccine was first introduced in Guangdong and Guangxi Provinces in July 2017; other provinces adopted the poultry vaccine by winter 2017–18. The vaccine coverage rate reported in November 2017 in Guangdong was 97.9% (282 million birds) among the target poultry population (7), which encompassed chickens, ducks, geese, quail, pigeons, and rare birds in captivity (8); however, the reported vaccine coverage varied in different provinces (8). Layers and breeders received 2 doses of the H5/H7 vaccine, whereas broilers sold within 70 days received 1 dose (8). To assess the effect of poultry vaccination on H7N9 prevalence in poultry and the subsequent effect on human zoonotic infection risk, we analyzed the temporal distribution of monthly H7N9 detection rates at LPMs and of human H7N9 cases in Guangdong Province during 2013– 2018. We estimated the effect of the bivalent H5/H7 vaccine using a Poisson regression model.