The so-called G4 variant of an H1N1 influenza virus, currently circulating among pigs, has genetic traces of several other dangerous pathogens, including the virus that caused the 2009 flu pandemic, according to a study published yesterday (June 29) in PNAS. In laboratory experiments, the researchers found that these G4 viruses were able to infect and replicate in human airway epithelial cells. Moreover, the study found that the viruses could be transmitted between ferrets.

“From the data presented, it appears that this is a swine influenza virus that is poised to emerge in humans,” University of Sydney evolutionary biologist Edward Holmes, who was not involved in the study, tells Science. “Clearly this situation needs to be monitored very closely.”

Ian Brown, the head of the virology department at Britain’s Animal and Plant Health Agency who peer-reviewed the paper, agrees. “It may be that with...

Researchers at China Agricultural University and colleagues found the concerning virus as part of a project that aims to identify flu strains with the potential to cause a pandemic if they leap from animals to humans. The team focused on pigs, analyzing some 30,000 nasal swabs collected from swine at slaughterhouses in China between 2011 and 2018, plus another 1,000 swabs from pigs seen at the university’s vet school for respiratory symptoms. Of the 179 influenza viruses identified, most were G4, a relatively new variant observed in pig populations. “G4 virus has shown a sharp increase since 2016, and is the predominant genotype in circulation in pigs detected across at least 10 provinces,” according to the study.

In addition to carrying genes from the 2009 pandemic flu, G4 viruses have genetic traces of avian influenza virus—particularly concerning because humans do not carry immunity to bird flu—and a viral strain that has a combination of avian, human, and pig influenza genes. Analyzing blood samples from pig farmers and 230 people who do not work with pigs, the researchers found antibodies against G4 viruses—signs of a previous infection—in 4.4 percent of the general population and more than 10 percent of swine farm workers, suggesting that the viral strain can infect humans.

See “Can a Vaccine Save the World’s Pigs from African Swine Fever?

Experts caution that the pandemic risk is unclear. “We just do not know a pandemic is going to occur until the damn thing occurs,” influenza researcher Robert Webster, who recently retired from St. Jude Children’s Research Hospital and was not involved in the study, tells Science. “Will this one do it? God knows.” Martha Nelson, an evolutionary biologist at the US National Institutes of Health’s Fogarty International Center who studies pig influenza viruses, adds: “The likelihood that this particular variant is going to cause a pandemic is low.”

Nevertheless, the study authors call for increased surveillance as well as efforts to develop a vaccine against the G4 virus, and others agree. “Right now we are distracted with coronavirus and rightly so. But we must not lose sight of potentially dangerous new viruses,” Kin-Chow Chang of Nottingham University in the UK who was also not involved in the research tells the BBC.

World Health Organization spokesman Christian Lindmeier noted at a press briefing in Geneva, “We will read carefully the paper to understand what is new,” according to Reuters. “It also highlights we cannot let our guard down on influenza and need to be vigilant and continue surveillance even in the coronavirus pandemic.”

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