Participants between 18 and 35 years of age had about 20 per cent positive rates of the virus in their blood, according to the study, indicating that the predominant G4 strain has acquired increased human infectivity.
Despite five years of extensive exposure of humans to the virus, there's no evidence of it spreading from one person to another, said Carl T. Bergstrom, a biologist at the University of Washington in the USA, who is unrelated to the study.
Tests revealed that G4 has already infected humans in China's Hebei and Shandong provinces, which have large swine populations.
"Pig farming is a massive industry in China and pigs can be important hosts from which novel influenza viruses may emerge", James Wood, Head of Veterinary Medicine at the University of Cambridge, told CNN.
"We just do not know a pandemic is going to occur until the damn thing occurs", said Robert Webster, an influenza investigator who recently retired from St. Jude Children's Research Hospital. And though G4 holds H1N1 genes, people who have received seasonal flu vaccines won't have any immunity.
She also shared her experience of contracting swine flu and said that it is the most hard thing she has gone through.
The surveillance of the virus was conducted between 2011 and 2018. Of the 179 influenza viruses identified, most were G4, a relatively new variant observed in pig populations. "It also highlights we can not let our guard down on influenza and need to be vigilant and continue surveillance even in the coronavirus pandemic".
Another pig farmer from eastern China told of his concern that African Swine Flu may creep back into the country and inflate an already precarious situation. Scientists say that pigs have been increasingly infected with the virus since 2016.
Researchers recommend that controlling G4 EA H1N1 in pigs and monitoring it in humans, "especially the workers in swine industry", should be implemented as soon as possible.
This proves the possibility of animals-to-human transmission of the virus, although there is still no evidence it can be transferred from human to human.
It can grow and replicate in human cells, the experiment showed. "Influenza can surprise us", Nelson says.
There is no evidence yet that G4 could spread from person to person, which perhaps is the most promising sign so far, said Carl Bergstrom, a professor of biology at the University of Washington.
"What the paper does do is something important for the epidemiological community: it points to a virus that we need to be keeping a careful eye on", the scientist tweeted about the new research.