G4 swine flu viruses in China described in the PNAS report have a mix of genes from influenza viruses found in humans, birds and pigs. Five genes of the G4 virus came from the 2009 H1N1 virus that caused the 2009 flu pandemic. Based on laboratory transmission studies in ferrets, G4 viruses can spread via direct contact or via respiratory droplets. CDC has tested a closely related G5 virus in its laboratories and observed transmission similar to what was reported in this study.

G4 viruses likely resulted from a process called “reassortment,” which occurs when two or more influenza viruses infect a single host and exchange genetic material. This can sometimes lead to the emergence of new influenza viruses in people or animals. Pigs have been identified as a sort of “mixing vessel” for reassortment to occur because pigs are susceptible to infection with influenza viruses found in pigs, birds and humans. The 2009 H1N1 pandemic arose from a reassortment event between pigs, birds and human influenza A viruses.