Evolutionary dynamics of avian influenza A H7N9 virus across five waves in mainland China, 2013-2017. 
Since its emergence in March 2013, novel avian influenza A H7N9 virus has triggered five epidemics of human infections in China.  This raises concerns about the pandemic threat of this quickly evolving H7N9 subtype for humans.  In this study, we evaluated all available genomes for H7N9 and H9N2 influenza A viruses.  Our assessment discovered that H7N9 of the 1st wave had the lowest nucleotide diversity, which then experienced substantial and rapid population expansion from a small founder population.  From the 2nd wave, their nucleotide diversity increased quickly, indicating that H7N9 viruses had acquired larger populations and mutations after their initial emergence in 2013.  After the phylogeographic divergence in the 2nd wave, although the HA and NA genes from different regions differed, compared to previous epidemics, the evolving H7N9 viruses in the 5th wave lost most of their previous clades.  The highly pathogenic avian influenza (HPAI) H7N9 viruses in the 5th wave clustered together, and clustered close to the low pathogenic avian influenza (LPAI) virus isolated from the Pearl River Delta in the 3rd and 4th waves.  This result supports the origin of HPAI H7N9 viruses was in the Pearl River Delta.  In the 5th wave, although both HPAI and LPAI H7N9 viruses were isolated from the Pearl River Delta, their HA and NA genes were phylogenetically distinct.