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H7N9 low pathogenic influenza viruses emerged in China in 2013 and mutated to highly pathogenic strains in 2017, resulting in human infections and disease in chickens. To control the spread, a bivalent H5 / H7 inactivated vaccine was introduced in poultry in September 2017. To monitor virus evolution and vaccine efficacy, we collected 53,884 poultry samples in China from February 2017 to January 2018. We isolated 252 low pathogenic H7N9 viruses, 69 highly pathogenic H7N9 viruses and one highly pathogenic H7N2 virus, from which two low pathogenic and 14 highly pathogenic strains were collected after the introduction of the vaccine. Genetic analysis of highly pathogenic strains revealed nine genotypes, one of which is predominant and widespread and contains strains that exhibit high virulence in mice. Additionally, some H7N9 and H7N2 viruses that carry duck virus genes are lethal in ducks. Therefore, although vaccination reduced H7N9 infections, increased virulence and expanded host range for ducks pose new challenges.

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H7N9 highly pathogenic influenza virus evolution virulence vaccine
Influenza A viruses are RNA viruses; Its genome consists of eight negative sense single-stranded RNA fragments that encode basic polymerase 2 (PB2), basic polymerase 1 (PB1), acid polymerase (PA), hemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), matrix (M) and non-structural (NS) proteins. Viruses are divided into different subtypes based on the antigenicity of their two surface glycoproteins, HA and NA. Currently, 16 different HA subtypes (H1-H16) and nine different NA (N1-N9) have been identified from bird species, and two additional HA subtypes (H17-18) and NA (N10-11) have been detected. in bats (Tong et al., 2012, Tong et al., 2013).

Influenza A viruses continually challenge the poultry industry and human health. The H1N1, H2N2, and H3N2 viruses have caused human influenza pandemics, and the H1N1 and H3N2 viruses still circulate widely in humans throughout the world. Several highly pathogenic H5 and H7 virus subtypes have caused outbreaks of avian influenza in poultry and wild birds in many countries since 1959 (Alexander and Brown, 2009, Chen, 2009, Fouchier et al., 2004, Li et al., 2010, Shi et al., 2017), which has disastrous consequences for the poultry industry. The H5N1 and H7N9 viruses have attracted a great deal of attention over the past two decades because they have caused not only problems for the poultry industry, but also serious human infections and deaths (WHO / GIP, 2018a, Wong and Yuen, 2006, Zhou et al. , 2013).

Since their emergence in February 2013, H7N9 influenza viruses have caused 1,567 human infections in mainland China, Hong Kong, Macao, and Taiwan, 615 of which were fatal (WHO / GIP, 2018b) as of June 24, 2018. The viruses were initially found in live poultry markets in various provinces (Chen et al., 2013, Shi et al., 2013, Zhang et al., 2013), and studies indicated that these 2013 H7N9 viruses isolated from Birds were not pathogenic for chickens and mice, and hardly replicated in ducks (Pantin-Jackwood et al., 2014, Zhang et al., 2013). Avian and human H7N9 viruses were able to bind to both avian and human-type receptors (Belser et al., 2013, Richard et al., 2013, Watanabe et al., 2013, Zhou et al., 2013), but the Human isolates were more lethal in mice and more transmissible in ferrets than isolates from birds (Zhang et al., 2013). In early 2017, some H7N9 HA mutants were detected from samples collected from live poultry markets in Guangdong province, and animal studies indicated that these mutants were highly pathogenic for chickens (Qi et al., 2018 , Shi et al., 2017). Although the index mutant H7N9 HA was not lethal in mice or ferrets, our previous study (Shi et al., 2017) and a study by Imai et al. (2017) revealed that H7N9 HA mutants could acquire additional mutations during their replication in ferrets or humans, and then become highly lethal in mammals and transmissible in ferrets by respiratory droplets. Yang et al. (2017) reported that 50% of human cases of H7N9 highly pathogenic influenza virus infection were fatal (since we did not test for virulence in chickens of all strains we obtained in this study, our use of "highly pathogenic "Throughout this text it simply means that the virus HA cleavage motif met the criteria for a virus of the