The emergence of human infection with a novel H7N9 influenza virus in China raises a pandemic concern. Chicken H9N2 viruses provided all six of the novel reassortant’s internal genes. However, it is not fully understood how the prevalence and evolution of these H9N2 chicken viruses facilitated the genesis of the novel H7N9 viruses. Here we show that over more than 10 y of cocirculation of multiple H9N2 genotypes, a genotype (G57) emerged that had changed antigenicity and improved adaptability in chickens. It became predominant in vaccinated farm chickens in China, caused widespread outbreaks in 2010–2013 before the H7N9 viruses emerged in humans, and finally provided all of their internal genes to the novel H7N9 viruses. The prevalence and variation of H9N2 influenza virus in farmed poultry could provide an important early warning of the emergence of novel reassortants with pandemic potential.

Since May 2014, highly pathogenic avian influenza H5N6 virus has been reported to cause six severe human infections three of which were fatal. The biological properties of this subtype, in particular its relative pathogenicity and transmissibility in mammals, are not known. We characterized the virus receptor-binding affinity, pathogenicity, and transmissibility in mice and ferrets of four H5N6 isolates derived from waterfowl in China from 2013-2014. All four H5N6 viruses have acquired a binding affinity for human-like SA␣2,6Gal-linked receptor to be able to attach to human tracheal epithelial and alveolar cells. The emergent H5N6 viruses, which share high sequence similarity with the human isolate A/Guangzhou/39715/2014 (H5N6), were fully infective and highly transmissible by direct contact in ferrets but showed less-severe pathogenicity than the parental H5N1 virus. The present results highlight the threat of emergent H5N6 viruses to poultry and human health and the need to closely track their continual adaptation in humans. O n 7 May 2014, the Chinese National Health and Family Planning Commission (NHFPC) announced the first human case of avian H5N6 influenza virus infection (1). Subsequently, three more human infections with H5N6 virus cases were reported in the winter of 2014-2015 (2, 3). Between 30 December 2015 and 2 January 2016, the NHFPC notified the World Health Organization of two additional human cases of avian H5N6 virus infection. All six human infections were presented as acute respiratory distress syndrome (ARDS) of which three were fatal. Five cases had a common history of contact with or exposure to poultry or livebird markets before disease onset (1-3), suggesting zoonotic transmission. Sequence analyses of the human H5N6 isolates indicated that the virus was derived from clade 2.3.4.4 avian H5N6 viruses that are circulating in poultry in China (1, 2, 4). Avian H5N6 influenza virus was first isolated from mallards in North America in 1975 (5). In China, H5N6 virus first emerged in 2010 and has since been extensively circulating in both domestic and wild birds (6-9). Recent surveillance data from the Ministry of Agriculture of China indicate that H5N6 viruses have become enzootic in domestic poultry. Unlike the worldwide distribution H5N2 and H5N8 viruses (10-12), prevailing H5N6 viruses appear to be largely confined to China and Laos (13). We recently characterized the novel H5N6 viruses in poultry (14); however, their zoonotic capability and characteristics are poorly understood. In the present study, we examined the emergent H5N6 virus for its genetic characteristics, receptor binding properties, pathogenicity, and transmissibility in mice and ferrets. MATERIALS AND METHODS Ethical

We previously demonstrated that H9N2 subtype avian influenza viruses (AIVs) isolated from 1994 to 2008 evolved into distinct antigenic groups (C, D, and E) and then underwent antigenic drift from commercial vaccines, causing a country-wide outbreak during 2010–2013. In this study, H9N2 AIVs isolated from chickens during 2009–2013 were antigenically analyzed by performing hemagglutination inhibition and neutralization assays using a panel of polyclonal antibodies. Our findings confirmed the antigenic drift of recent H9N2 viruses from the commercial vaccine and showed that most of these antigenic variants form a novel HI antigenic group, F, with a few belonging to groups D and E. Slight antigenic variation was observed in group F viruses. Genetic analysis of amino acid sequences deduced from hemagglutinin (HA) gene sequences indicated that 9 of 15 mutations predominant in the 2009–2013 viruses can be mapped to known antigenic sites, which might be responsible for the novel antigenicity of group F. These antigenic changes make it necessary to modify the influenza vaccine to ensure efficient protection. A vaccine candidate, Ck/HeB/YT/10, was selected and provided significant protection against viruses from different antigenic groups in terms of reduction in virus shedding, suggesting broad cross-reactivity. Taken together, our results indicate that the H9N2 chicken influenza viruses in China have evolved from distinct antigenic groups into a novel group F that became dominant during the country-wide outbreak and now seems to be undergoing new antigenic divergence. Systematic surveillance and timely updating of vaccine strains are important for viral prevention and control in the future.

To prevent H9N2 avian influenza virus infection in chickens, a long-term vaccination program using inactivated vaccines has been implemented in China. However, the protective efficacy of inactivated vaccines against antigenic drift variants is limited, and H9N2 influenza virus continues to circulate in vaccinated chicken flocks in China. Therefore, developing a cross-reactive vaccine to control the impact of H9N2 influenza in the poultry industry remains a high priority. In the present study, we developed a live cold-adapted H9N2 influenza vaccine candidate (SD/01/10-ca) by serial passages in embryonated eggs at successively lower temperatures. A total of 13 amino acid mutations occurred during the cold-adaptation of this H9N2 virus. The candidate was safe in chickens and induced robust hemagglutination-inhibition antibody responses and influenza virus–specific CD4+ and CD8+ T cell immune responses in chickens immunized intranasally. Importantly, the candidate could confer protection of chickens from homologous and heterogenous H9N2 viruses. These results demonstrated that the cold-adapted attenuated H9N2 virus would be selected as a vaccine to control the infection of prevalent H9N2 influenza viruses in chickens.

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