The current H9N2 vaccine strain did not antigenically match the circulating Fujian-like viruses, which caused a H9N2 outbreak across almost the entire country soon afterwards. The novel H9N2 AIV immune escape variant had been circulating in Fujian province, China since early 2020. Hence, an improved understanding of the molecular basis of antigenic differences between two related strains could ultimately facilitate the selection of more effective vaccine components to control the circulation of influenza in poultry. While one or two amino acid changes can cause sufficient antigenic drift in some situations, simultaneous substitutions linked with unselected mutations at several sites is most generally observed. These residues identified by mAbs were important for understanding the viral antigenicity however, many residues are completely conserved among circulating viruses. In previous studies, a certain number of escape mutations in the HA gene of H9N2 virus have been confirmed by using a common monoclonal antibodies (mAbs) escape mutant method, and several residues were assigned to two discrete antigenic sites, ‘H9-A’ and ‘H9-B antigenic epitopes. Understanding the antigenic property of circulating viruses is essential for updating matched vaccine strains to control the circulation of H9N2 in poultry. Therefore, HA is the focus of influenza virus surveillance and a major component of currently licensed vaccines. Antigenic drift is mediated by the gradual accumulation of mutations in haemagglutinin (HA) glycoprotein, which results in viruses that escape from prior antibodies produced by natural infection or vaccination. Although conventional inactivated vaccines were used in many countries for disease control, the virus continues to circulate in vaccinated chicken farms, which is possibly due to antigenic drift. Due to sporadic cases of human infection, and by providing reassortant internal genes to human-infecting subtypes such as H5N1, H5N6, H7N9, and H10N8, H9N2 AIV is posing a significant threat to public health with potential pandemic risk. Available evidence has demonstrated that H9N2 viruses isolated naturally from poultry have acquired the ability to preferentially bind to the human-type receptor. H9N2 AIV has gradually become the most prevalent subtype of AIVs in mainland China since 1994. H9N2 AIVs are widely spread in poultry worldwide and causes severe economic losses to poultry industries. However, the pandemic threat posed by other subtypes of avian influenza viruses, especially the H9 subtype, should not be overlooked. The focus of global pandemic preparedness is H5 and H7 AIVs. In recent year, novel avian influenza viruses (AIVs) have emerged as a major threat to animal and human. The Creative Commons Public Domain Dedication waiver ( ) applies to the data made available in this article, unless otherwise stated in a credit line to the data. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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