G4 swine flu unlikely health risk, CDC says

Current evidence suggests it is extremely unlikely that the public will be infected by the G4 swine influenza virus, a subtype of the H1N1 swine flu virus that caused the 2009 influenza pandemic, the Chinese Center for Disease Control and Prevention said on Friday.

On June 29, Chinese scientists published a study in the journal Proceedings of the National Academy of Sciences that said they had found a group of viruses－referred to as the "Eurasian avian-like H1N1 swine influenza viruses"－in Chinese pigs. They found the viruses could infect humans and therefore are of potential pandemic concern.

The study said the group of viruses includes six strains, labeled from G1 to G6, with the G4 variant being the most worrisome as it exhibits "all the essential hallmarks of being highly adapted to infect humans, and should be paid close attention to".

The China CDC said in a statement there have been no reports of this group of viruses spreading from person to person, a prerequisite for a pandemic. The World Health Organization and the United States are reported to be closely monitoring the virus' development.

"The current G4 virus does not increase the risk of an influenza pandemic, and the research results also do not suggest the virus poses an immediate threat to public health," it said.

Since 2010, China's influenza monitoring network has reported 13 human cases of EA-H1N1 viral infection, three of which were G4 infections, the CDC said. China tests over 400,000 samples on average each year.

Given the low number of cases, the China CDC said "the probability of the public being infected by the virus is extremely low." Maintaining good personal hygiene, getting annual flu shots and reducing direct contact with livestock and wild animals are suggested protective measures.

Pork products are also safe to consume so long as they are purchased from credible vendors and are cooked properly, it said.

Since 2009, the EA-H1N1 viruses have been "reassorting" and evolving with the pandemic-causing H1N1, it said. Reassortment is a process in which two or more influenza viruses infect a single host and swap genetic material, leading to the emergence of new influenza viruses.

"The EA-H1N1 are not recently discovered viruses. They have been in China's swine populations for years, and the G4 strain has become the predominant genotype in Chinese pigs since 2016," the China CDC said.

The G4 swine flu virus can infect cells of the human upper respiratory system in vitro, and animal testing on ferrets shows it can be spread via respiratory droplets and direct contact, it said.

However, neither in vitro tests nor animal tests are enough to prove that the virus can spread in the same way among humans, it added.

Yang Hanchun, a swine disease expert at China Agricultural University, said in a statement posted by the Ministry of Agriculture and Rural Affairs on Friday that the G4 virus has been monitored by China and the WHO since 2011.

"Key authors of the study said the G4 swine flu virus cannot replicate effectively in human bodies and cause disease," he said. "The pig farmworkers involved in the study also did not showcase influenza symptoms."

While the study published in PNAS said about 10 percent of the 338 swine workers and 4.4 percent of the 230 other people who participated in it had antibodies against the EA-H1N1 viruses, Yang said the sample size was too small to yield any reliable conclusions about the virus' contagiousness.

Carl Bergstrom, a professor of biology at the University of Washington, posted on Twitter that the G4 virus has been very common in pigs since 2016.

"There's no evidence that G4 is circulating in humans, despite five years of extensive exposure. That's the key context to keep in mind," he wrote.