Abstract
An influenza A virus that originated in pigs caused a pandemic in 2009. The sialidase activity of the neuraminidase (NA) of previous pandemic influenza A viruses are stable at low pH (≤5). Here, we identified the amino acids responsible for this property. We found differences in low-pH stability at pH 5.0 among pandemic (H1N1) 2009 viruses, which enhanced the replication of these viruses. Low-pH-stable NA enhancement of virus replication may have contributed to the rapid worldwide spread and adaptation to humans of pandemic (H1N1) 2009 viruses during the early stages of the 2009 pandemic.