Emerg Infect Dis

We characterized 55 influenza A(H9N2) viruses isolated in Pakistan during 2014-2016 and found that the hemagglutinin gene is of the G1 lineage and that internal genes have differentiated into a variety of novel genotypes. Some isolates had up to 4-fold reduction in hemagglutination inhibition titers compared with older viruses. Viruses with hemagglutinin A180T/V substitutions conveyed this antigenic diversity and also caused up to 3,500-fold greater binding to avian-like and >20-fold greater binding to human-like sialic acid receptor analogs. This enhanced binding avidity led to reduced virus replication in primary and continuous cell culture. We confirmed that altered receptor-binding avidity of H9N2 viruses, including enhanced binding to human-like receptors, results in antigenic variation in avian influenza viruses. Consequently, current vaccine formulations might not induce adequate protective immunity in poultry, and emergence of isolates with marked avidity for human-like receptors increases the zoonotic risk.2018Wellcome Trust/United KingdomBBS/E/I/00001025/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/00001708/Biotechnology and Biological Sciences Research Council/United KingdomBB/L018853/1/Biotechnology and Biological Sciences Research Council/United KingdomBB/N503563/1/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/0000198/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/00007034/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/00007035/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/00007038/Biotechnology and Biological Sciences Research Council/United KingdomBBS/E/I/00007039/Biotechnology and Biological Sciences Research Council/United KingdomFC001030/Cancer Research UK/United KingdomFC001030/Medical Research Council/United KingdomFC001030/Wellcome Trust/United Kingdom30561311PMC6302589656