On June 30, 2020, several news outlets, including BBC News, reported on a new variant of the H1N1 swine flu that “has the potential to become a pandemic.” These reports stem from a June 29 scientific paper in the journal Proceedings of the National Academy of Sciences (PNAS). Using a trove of data collected from pigs in China, along with animal experiments and epidemiological observations, the researchers concluded that a variant of the virus responsible for the 2009 swine flu pandemic is increasingly prevalent in pigs and can be transferred to humans. It has, they say, the potential to cause a deadly human pandemic.

Broadly speaking, two things are required before an animal-derived virus can cause a pandemic. First, the virus — though hosted by an animal such as a pig or a bird — must evolve the ability to transfer to, and replicate inside of, a human body. Second, the virus needs to be capable of spreading from one human to another. As of this date, researchers have observed this strain in humans who work in proximity to pigs, but there is no evidence of the latter human-to-human-spread.

What Is Swine Flu?

The 2009 Swine Flu pandemic was caused by a form of influenza A virus — (H1N1)pdm09 — that formed, according to a review in the journal Scientific Reports, “as a result of re-assortment between avian, human, and swine influenza viruses.” Though the virus spread rapidly around the world, it was not particularly lethal. This lack of lethality was, in part, thanks to the fact that, according to the Centers for Disease Control and Prevention (CDC), “nearly one-third of people over 60 years old had antibodies against this virus, likely from exposure to an older H1N1 virus earlier in their lives.”

Scientists monitor the genetic drift and evolution of myriad influenza viruses in an effort to create an effective flu shot each year, but they have also been monitoring variations of H1N1 in animal populations due to the risk that a more virulent form of this influenza virus could evolve. The June 2020 report of PNAS represents this latter effort.

What’s This New Swine Flu?

In general, RNA viruses like influenza mutate rapidly compared with DNA viruses. Viruses, which are composed primarily of short bits of genetic material, can sometimes combine with other viruses and mix genetic material between them. Over time, mutations that are advantageous to the virus become dominant, occasionally giving it new properties that could make it capable of surviving in other host organisms such as humans.

From 2011 to 2018, the PNAS researchers collected nearly 30,000 nasal swabs from pigs and identified over 100 strains of swine influenza. Their work demonstrated that, starting in 2016, a modified form of H1N1 — named “G4 EA H1N1”  — became the dominant strain in these pigs. This is potentially troubling, the researchers argued, because the so-called “G4 genotype” of H1N1 is capable of binding “to human-type receptors,” can replicate easily in human airway cells, and can be transferred via the air.

Indeed, the researchers found that 10 percent of individuals (of 338 sampled) who work close to pigs tested positive for G4 EA H1N1, indicating that the virus “has acquired increased human infectivity.” This is evidence that this new swine flu can jump between pig and human, but it is not evidence of transfer between humans.

Will This New Swine Flu Become the Next Pandemic?

Because the virus can readily transfer itself from pig to human, according to the PNAS researchers, the fear is that the virus has had, and will continue to have, repeated opportunities to adapt while harbored within the body of an infected human. Such adaptations could lead to the ability for human-to-human transfer of the virus either through direct contact or airborne transmission. If that did occur, the risk of a pandemic, according to the authors, is further enhanced by the fact that the world population has very low natural immunity to G4-type influenza viruses.

The CDC’s Fauci, in a U.S. Senate hearing on the COVID-19 epidemic, told lawmakers that “the possibility that you might have another swine flu-type outbreak as we had in 2009” is real, but that “It’s something that still is in the stage of examination” and “not an immediate threat.” In a statement given to outlets including CNN and the BBC, a World Health Organization spokesperson said:

Eurasian avian-like swine influenza virus are known to be circulating in the swine population in Asia and to be able to infect humans sporadically. 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 [PNAS] paper to understand what is new.

So what’s next? According to the PNAS team, measures exist that could greatly reduce the chance of a pandemic. “Controlling the prevailing G4 EA H1N1 viruses in pigs and close monitoring in human populations, especially the workers in [the] swine industry,” they wrote, “should be urgently implemented.”