Emergence of novel avian origin H7N9 viruses after introduction of H7-Re3 and rLN79 vaccine strains to China.

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.