In the present study, we determined by RT-PCR the genomic sequences of an H3N2 influenza A virus, denoted A/Swine/GuangDong/z5/2003(H3N2), isolated from a pig in December 2003 in Guangdong Province, south China. We then performed
VX-809 analyses to determine the genetic association between this swine H3N2 virus and human and swine H3N2 predominating at different periods. We found that the HA, NA and other gene segments of A/Swine/GuangDong/z5/2003 exhibited higher genetic similarity to the Moscow/99-like human H3N2 viruses than the European swine H3N2 viruses during the
period from 1999 to 2002. The antigenic relatedness of the HA protein of A/Swine/GuangDong/z5/2003 is also much closer to that of 99-like human H3N2 viruses than the European swine H3N2 viruses in 1999–2002. These results suggest that this swine H3N2 virus may have been transmitted from humans to pigs, possibly being driven by human immune pressure induced by either natural infection of human H3N2 viruses or by immunization with A/Moscow/10/99-based human influenza vaccine.
