identify a recently emerged genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 virus, which bears 2009 pandemic (pdm/09) and triple-reassortant (TR)-derived internal genes and has been predominant in swine populations since 2016. Similar to pdm/09 virus, G4 viruses bind to human-type receptors, produce much higher progeny virus in human airway epithelial cells, and show efficient infectivity and aerosol transmission in ferrets. Moreover, low antigenic cross-reactivity of human influenza vaccine strains with G4 reassortant EA H1N1 virus indicates that preexisting population immunity does not provide protection against G4 viruses. Further serological surveillance among occupational exposure population showed that 10.4% (35/338) of swine workers were positive for G4 EA H1N1 virus, especially for participants 18 y to 35 y old, who had 20.5% (9/44) seropositive rates, indicating that the predominant G4 EA H1N1 virus has acquired increased human infectivity.

The study described pigs as important “mixing vessels” for pandemic influenza viruses, adding that one particular virus, the “G4” strain of H1N1, boasted “all the essential hallmarks of a candidate pandemic virus.”

The newly-identified G4 virus is a recombination of the H1N1 strain identified in 2009 – which led to a major quarantine in China at the time – and a previous strain of swine influenza.

In animal studies, including on ferrets whose immune response can often be quite similar to humans, G4 was found to be highly infectious, replicating in implanted human cells and causing more serious symptoms than expected.

Worryingly, pig factory workers showed elevated G4 levels in their blood, while any immunity in human cells following exposure to seasonal flu provided no protection against G4.

More than one in 10 workers who handled swine had already been infected with G4, according to antibody blood tests conducted by the study authors. To make matters worse, some 4.4 percent of the general population tested had already been exposed, showing that the virus had passed from animal to human – but has yet to transit human to human.