Since its emergence in 2013, the H7N9 low-pathogenic avian influenza virus (LPAIV) has been circulating in domestic poultry in China, causing five waves of human infections. A novel H7N9 highly pathogenic avian influenza virus (HPAIV) variant possessing multiple basic amino acids at the cleavage site of the hemagglutinin (HA) protein was first reported in two cases of human infection in January 2017. More seriously, those novel H7N9 HPAIV variants have been transmitted and caused outbreaks on poultry farms in eight provinces in China. Herein, we demonstrate the presence of three different amino acid motifs at the cleavage sites of these HPAIV variants which were isolated from chickens and humans and likely evolved from the preexisting LPAIVs. Animal experiments showed that these novel H7N9 HPAIV variants are both highly pathogenic in chickens and lethal to mice. Notably, human-origin viruses were more pathogenic in mice than avian viruses, and the mutations in the PB2 gene associated with adaptation to mammals (E627K, A588V, and D701N) were identified by next-generation sequencing (NGS) and Sanger sequencing of the isolates from infected mice. No polymorphisms in the key amino acid substitutions of PB2 and HA in isolates from infected chicken lungs were detected by NGS. In sum, these results highlight the high degree of pathogenicity and the valid transmissibility of this new H7N9 variant in chickens and the quick adaptation of this new H7N9 variant to mammals, so the risk should be evaluated and more attention should be paid to this variant. Due to the recent increased numbers of zoonotic infections in poultry and persistent human infections in China, influenza A(H7N9) virus has remained a public health threat. Most of the influenza A(H7N9) viruses reported previously have been of low pathogenicity. Now, these novel H7N9 HPAIV variants have caused human infections in three provinces and outbreaks on poultry farms in eight provinces in China. We analyzed the molecular features and compared the relative characteristics of one H7N9 LPAIV and two H7N9 HPAIVs isolated from chickens and two human-origin H7N9 HPAIVs in chicken and mouse models. We found that all HPAIVs both are highly pathogenic and have valid transmissibility in chickens. Strikingly, the human-origin viruses were more highly pathogenic than the avian-origin viruses in mice, and dynamic mutations were confirmed by NGS and Sanger sequencing. Our findings offer important insight into the origin, adaptation, pathogenicity, and transmissibility of these viruses to both poultry and mammals.

No avian H7N9 outbreaks have occurred since the introduction of H7N9 inactivated vaccine in the fall of 2017. However, H7N9 is still prevalent in poultry. To surveil the prevalence, genetic characteristics, and antigenic changes of H7N9, over 7000 oropharyngeal and cloaca swab specimens were collected from live poultry markets and farms in 15 provinces of China from 2017 to 2019. A total of 85 influenza virus subtype H7N9 strains were isolated and 20 representative strains were selected for genetic analysis and antigenicity evaluation. Results indicated the decreased prevalence of low-pathogenic H7N9 strains while highly-pathogenic H7N9 strains became dominated since the introduction of vaccine. Phylogenetic analysis showed that strains from 2019 formed an independent small branch and were genetically distant to strains isolated in 2013–2018. Analysis of key amino acid sites showed that the virus strains may adapt to the host environment evolutionally through mutation. Our analysis predicted additional potential glycosylation sites for HA and NA genes in the 2019 strains. Sequence analysis of HA gene in strains isolated from 2018 to 2019 showed that there were an increased nucleotide substitution rate and an increased mutation rate in the first and second nucleotides of coding codons within the open reading frame. The hemagglutination inhibition (HI) assay showed that H7-Re1 and H7-Re2 exhibited a lower HI titer for isolates from 2019, while H7-Re3 and rLN79 showed a high HI titer. The protective effect of the vaccine decreased after 15 months of use. Overall, under vaccination pressure, the evolution of influenza virus subtype H7N9 has accelerated. Supplementary Information The online version contains supplementary material available at 10.1007/s12250-021-00383-x.

In early 2021, roughly 6 months after the H7N9 H7‐Re3 and H7N9 rLN79 vaccine strains were introduced into China, we monitored a number of H7N9 subtype avian influenza viruses, which could have escaped vaccine‐induced immunity in live poultry markets (LPMs) in Yunnan, Hebei, Shanxi and Guangdong provinces, China. To investigate whether these viruses were a novel H7N9 variant of highly pathogenic avian influenza (HPAI) virus and whether they had the potential for further spread, we characterized the genetic evolution, antigenic divergence and pathogenicity of the viruses in the context of vaccine immunity. The results show further diversification in the HA gene of newly isolated HPAI H7N9 viruses compared with antigenic variants that emerged after the period of 2017–2019. There were clear antigenic differences between current vaccines and these viruses, and SPF broilers under vaccine protection could not resist virus challenges. Our study demonstrates that the current vaccine has insufficient protective capacity against the novel H7N9 variants under experimental conditions. A novel H7N9 immune escape virus has emerged. Faced with potential outbreaks, we should strengthen surveillance and update vaccine strains.

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