Researchers in China have identified an influenza virus called G4 that can infect both pigs and humans. While G4 is not yet able to spread from person to person, the scientists say that it has “all the essential hallmarks” of a future pandemic virus.

Scientists have likened pigs to “mixing vessels” for generating pandemic influenza viruses because they host both mammalian and avian flu viruses.

When different strains of a virus occupy the same animal, they can swap genes to create new strains with the potential to infect new hosts.

Research led by Honglei Sun at China Agricultural University (CAU) in Beijing has identified such a strain in pigs that has already begun to infect humans.

Called G4, it incorporates genes from three distinct influenza strains:

• a strain similar to viruses present in European and Asian birds
• a North American strain that has genes from avian, human, and pig influenza viruses
• the H1N1 strain that researchers first detected in the United States and that caused the 2009 swine flu pandemic

There is currently no evidence that G4 can pass from person to person. However, the presence of genes from the H1N1 pandemic strain suggests that it might develop this ability in the future.

Between 2011 and 2018, the research team analyzed about 30,000 nasal swabs taken from pigs at slaughterhouses in 10 Chinese provinces.

They also analyzed 1,000 swabs from pigs with respiratory symptoms that had received treatment at CAU’s veterinary teaching hospital.

The researchers identified a total of 179 swine influenza viruses, including G4, which began to predominate in the samples from 2016 onward.

Describing their findings in the journal Proceedings of the National Academy of Sciences of the United States of America, the researchers say that G4 has “all the essential hallmarks of a candidate pandemic virus.”

They also detected antibodies to the virus in the blood of people who work at pig farms.

Out of 338 workers who underwent testing for G4, 35 (10.4%) received positive results. The infection rate was higher among younger workers aged 18–35, with nine out of 44 (20.5%) testing positive.

A household survey found antibodies to G4 in 4.4% of 230 people who underwent testing.

The scientists write that this level of infectivity “greatly enhances the opportunity for virus adaptation in humans and raises concerns for the possible generation of pandemic viruses.”

In the lab, the researchers showed that the virus was able to infect cultures of human epithelial cells that line the airways of the lungs.

The virus also had the ability to infect ferrets, which researchers often use to model human influenza, and to transmit from animal to animal via tiny airborne droplets called aerosols.

Infectious diseases resulting from pathogens that have jumped from one host species to another are known as zoonoses.

Prof. James Wood, head of the Department of Veterinary Medicine at the University of Cambridge in the United Kingdom, told the Science Media Centre in London:

“The work comes as a salutary reminder that we are constantly at risk of new emergence of zoonotic pathogens and that farmed animals, with which humans have greater contact than with wildlife, may act as the source for important pandemic viruses.”

Dr. Alice Hughes from the Centre for Integrative Conservation at the Xishuangbanna Tropical Botanical Garden in Yunnan, China, noted that intensive farming practices in Asia might promote the spread of zoonotic viruses.

“Hygiene standards and feeds, including hormones and steroids across Asia, are likely to be contributory factors to compromised immune systems and the potential of viruses to spread,” she said. “Pork and poultry are also very popular across Asia, so there are huge numbers of the animals in the region.”

The farming of half the world’s population of 677.6 million pigs takes place in China.

In their paper, the scientists warn that existing flu vaccines are unlikely to protect human populations from G4.

Martha Nelson, an evolutionary biologist at the National Institutes of Health’s Fogarty International Center, who studies pig influenza viruses, said that in an ideal world, scientists would create a G4 vaccine in preparation for a possible outbreak.

Doing this would involve substantial funding, however.

She told the journal Science: “We need to be vigilant about other infectious disease threats even as COVID is going on because viruses have no interest in whether we’re already having another pandemic.”