New analysis in the journal Proceedings of the National Academy of Science (PNAS) describes a group of H1N1 swine influenza viruses that have “the essential hallmarks of being highly adapted to infect humans” and which are therefore of potential pandemic concern. While independent experts argue against the chances of a global outbreak, PNAS continues to stress the need for increased monitoring of swine health in pig farms across the high-risk region. In a statement, China’s agriculture ministry maintains that the study has been interpreted by the media “in an exaggerated and nonfactual way.”

This group of viruses, referred to as “G4” Eurasian (EA) avian-like H1N1 viruses, has been spreading in pigs in China since 2016 and has become the predominant genotype found in Chinese pigs, the US Centers for Disease Control and Prevention (CDC) highlights.

Weighing in on these new findings, independent scientists have argued against G4 viruses’ capacity to spread at the same rate as the COVID-19 pathogen. Carl Bergstrom, Professor of Biology at the University of Washington, US, took to Twitter to comment, “This is not a new virus; it’s 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.”

Dr. Angela Rasmussen, a virologist at Columbia University’s public health school, similarly took to Twitter to echo this sentiment. “Our understanding of what is a potential pandemic influenza strain is limited. Sure, this virus meets a lot of the basic criteria but it’s not for sure going to cause a hypothetical 2020 flu pandemic, or even be a dominant strain in humans.”

It is crucial to flag that there are no reports of G4 viruses spreading from person-to-person, a characteristic that is required in order for a pandemic to occur. Furthermore, G4 viruses have not been detected in pigs or people in the US. The CDC is presently taking action to monitor and prepare against this emerging public health threat, including:

• Coordinating with public health partners in China, including requesting a virus sample.

• Assessing the risk of the virus causing a pandemic using CDC’s Influenza Risk Assessment Tool (IRAT).

• evaluating whether an existing candidate vaccine virus (CVV) against a closely related flu virus (called “G5”) would protect against this virus.

• Creating a new CVV specific to G4 viruses, if needed.

• Studying whether existing flu antiviral drugs offer protection against this group of viruses.

According to the PNAS report, however, these viruses do have the right characteristics for causing infections in people, including the ability to grow well in human lung cells and to spread by respiratory droplets and direct contact in an animal model. While only two human infections with G4 viruses have previously been reported, this study found that around 10 percent of swine workers from whom blood samples were taken in China had evidence of prior infection with G4 viruses, suggesting that human infection is more common than previously thought.

Livestock as a melting pot for pathogens
G4 swine flu viruses in China described in the PNAS report have a mix of genes from influenza viruses found in humans, birds and pigs. Five genes of the G4 virus came from the 2009 H1N1 virus that caused the 2009 flu pandemic.

G4 viruses likely resulted from a process called “reassortment,” which occurs when two or more influenza viruses infect a single host and exchange genetic material. This can sometimes lead to the emergence of new influenza viruses in people or animals. Pigs have been identified as a sort of “mixing vessel” for reassortment to occur because pigs are susceptible to infection with influenza viruses found in pigs, birds and humans. The 2009 H1N1 pandemic, for instance, arose from a reassortment event between pigs, birds and human influenza A viruses.

The PNAS study showed that among 338 swine workers whose blood was tested for antibodies, about 10 percent had evidence of past infection with G4 viruses. A higher rate was observed among 18- to 35-year-old swine workers. Study authors caution that continued circulation of these viruses in pigs and exposure to humans may allow for additional reassortment events to occur and that these viruses and infections should be monitored closely.

The CDC reports that most people may lack immunity against G4 viruses, and despite seasonal flu vaccines protecting against the 2009 H1N1 virus, G4 viruses are different enough that seasonal flu vaccines would be unlikely to provide protection or prevent onward human-to-human transmission.

A prototype candidate vaccine virus (CVV) from the closely related EA avian-like H1N1 G5 swine flu virus was originally developed by the World Health Organization (WHO) Collaborating Center at the China CDC. A comparison of the genome of this virus to that of G4 viruses reveals that these two viruses are similar. Studies are planned to assess cross-reactivity between this vaccine virus and G4 viruses.

Scientific and industrial communities alike have pointed to the propagation of plant-based diets as a significant way to combat zoonotic pandemics. Meanwhile, meat shortages throughout the ongoing COVID-19 pandemic have been noted as having an upswing effect on the appetite for meatless offerings.

Meat shortages were also a hallmark of the previous year, which was marked by Asia’s efforts to combat the fast-spreading outbreak of African swine fever (ASF), which has wiped out herds of pigs across the continent, fueling concerns about food security. Currently, it is having a resurgence in Nigeria’s pork industry.