Generation of highly productive and mammalian nonpathogenic recombinant H9N2 avian influenza viruses by optimization of 3’end promoter and NS genome

•Recombinant H9N2 virus with weak PB2 activity can be rescued by promoter optimization.•Strong 3’end promoter of PB2 and PB1 genomes can compensate weak PB2 activity.•The 3’end promoter optimization improves the productivity of recombinant H9N2 viruses.•NS1 gene plays common role in mammalian and ch...

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