A team of researchers in China has identified an emerging influenza virus that might pose a threat in a future flu season. But the new virus so far cannot jump from person to person, a key attribute for becoming a pandemic in humans.
The paper, published in the Proceedings of the National Academy of Sciences, shows that a flu virus called G4 started to become more common in pigs in 2016. For now, the World Health Organization and the U.S. Centers for Disease Control and Prevention are watching G4 closely.
“People in my field—infectious disease research—are alert but not alarmed,” University of Queensland virologist Ian Mackay writes in the Conversation. “New strains of flu do pop up from time to time and we need to be ready to respond when they do, watching carefully for signs of human-to-human transmission.”
Starting in 2013, the researchers tested thousands of pigs across ten Chinese provinces for flu infections. They found 179 kinds of swine influenza virus, Nature reports. Starting in 2016, the G4 virus became the most prevalent in their sample.
Evolutionary biologist Martha Nelson, who was not involved in this study, tells Jon Cohen at Science magazine that while the finding is interesting, the team tested too few animals to get an accurate snapshot of the 500 million pigs that live in China. Then again, “influenza can surprise us,” Nelson adds, and there’s a that risk researchers could neglect influenza and other threats because of the COVID-19 pandemic.
G4 is specialized for infecting the cells in pig lungs, which are slightly different than human lung cells. Their surfaces are similar enough, however, that a swine flu virus can jump from pig to person from time to time. When that happens, the virus typically doesn’t spread any further. Sometimes, though, it’s able to pass from human to human, which is what happened in 2009 when an H1N1 swine flu spread.
“We just do not know a pandemic is going to occur until the damn thing occurs,” Robert Webster, an influenza investigator who recently retired from St. Jude Children’s Research Hospital, tells Science. “Will this one do it? God knows.”
G4 contains a mix of familiar and novel genes. Five of its genetic segments come from the 2009 swine flu, while the rest come from two lineages: the varieties of avian flu that infect European and North American birds and another form of swine flu that didn’t cause the 2009 pandemic, according to Science.
So far, only two active cases of G4 flu have been recorded in humans. Neither person passed the disease to others. The new report shows that out of 338 swine industry workers tested for antibodies to the new flu virus, a sign that they’d been infected with it in the past, 35 had the antibodies.
The virus was also able to infect and pass between ferrets, which are used to study influenza because they have similar lung cells and flu symptoms to humans. The researchers argue that this shows a worrying potential for transmission in humans.
"Right now we are distracted with coronavirus, and rightly so. But we must not lose sight of potentially dangerous new viruses," Kin-Chow Chang, an influenza expert at Nottingham University, tells the BBC’s Michelle Roberts. Chang says the G4 virus is not an immediate threat, but that “we should not ignore it.”
The CDC responded to the new research on Thursday, emphasizing that the G4 flu virus had not been seen in the U.S. and has not been passed from person to person. But the organization plans to work with Chinese researchers to study a sample of the virus, assess its risk and evaluate whether it can and should be included in the next annual flu vaccine.
"Twice a year during the influenza vaccine composition meetings, all information on the viruses is reviewed and the need for new candidate vaccine viruses is discussed. We will carefully read the paper to understand what is new,” a World Health Organization spokeswoman told the BBC. "We cannot let down our guard on influenza; we need to be vigilant and continue surveillance even during the COVID-19 pandemic."