To cause pandemics, zoonotic respiratory viruses need to adapt to replication in and spread between humans, either via (indirect or direct) contact or through the air via droplets and aerosols. To render influenza A viruses transmissible via air, three phenotypic viral properties must change, of which receptor-binding specificity and polymerase activity have been well studied. However, the third adaptive property, hemagglutinin (HA) acid stability, is less understood.
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