This Swine Flu Strain Has All The Hallmarks Of A Pandemic Threat

Researchers identify a strain of influenza virus in pigs that could cause a global pandemic. A new study has identified an influenza virus circulating in pigs that shows multiple traits associated with pandemic potential. Known as H1N2, the virus belongs to a lineage of flu viruses that originated in birds, adapted to pigs, and has now been detected in humans.

According to researchers, this strain—called 1C H1N2—has the ability to infect human airway cells, transmit between individuals, and escape most people’s existing immunity. These characteristics place it in a high-risk category under a pandemic risk framework published last year in Nature Communications . Influenza A viruses are not exclusive to humans.



They also infect birds, pigs, horses, and other animals. In pigs, three main subtypes circulate: H1N1, H1N2, and H3N2. These viruses are further divided into genetic lineages based on their evolutionary history.

The 1C lineage emerged in Europe in 1979, when an avian flu virus crossed into pigs and contributed genetic material to a new strain. Since then, the 1C lineage has remained largely confined to swine populations in Europe and Asia—until recently. Between 2011 and 2024, viruses from the 1C lineage have been detected in humans at least 29 times, with almost half of those cases occurring since 2021.

Most of these cases appear to be isolated spillover events from pigs, but their increasing frequency is cause for concern. Surveillance data also suggest that individuals who work with pigs are more likely to have been exposed to these viruses, underscoring the potential for transmission at the human–animal interface. Human cases of swine-origin 1C H1N2 influenza have been increasing since 2011.

One of the most striking findings from the new study is how little immunity most people have to the 1C H1N2 virus. In blood samples collected from U.S.

adults, only about one-third showed any detectable neutralizing antibodies, and most of those were from individuals born before 1950. This limited protection appears to stem from residual immunity to the 1918-like viruses that circulated early in the 20th century. More recent generations show almost no immune recognition of 1C H1N2.

Current seasonal flu vaccines also offer little to no cross-protection, according to the study. In laboratory tests, the 1C H1N2 virus displayed several traits associated with the ability to spread among humans. Like the 2009 pandemic H1N1 virus, it binds preferentially to receptors found in the human upper respiratory tract—an important trait for human-to-human transmission.

It replicated efficiently in cultured human airway cells and remained stable in aerosol-like droplets across a range of humidity levels, mimicking the conditions respiratory viruses encounter in real-world settings. The virus also showed high neuraminidase activity, a feature that has been linked to increased transmissibility. Neuraminidase is a key protein on the virus’s surface that helps it escape from infected cells to infect new cells.

To evaluate the virus’s ability to cross species, researchers conducted transmission experiments using pigs and ferrets—a standard model for studying influenza in humans. Infected pigs were housed near uninfected ferrets with no direct contact. While the ferrets showed no symptoms, all of them developed antibodies against the virus after exposure, indicating successful transmission.

Notably, even ferrets that had prior immunity to the 2009 H1N1 pandemic strain became infected, suggesting that existing human immunity may not prevent spread of the 1C virus if it were to emerge more widely. Human cases of swine-origin 1C H1N2 have appeared in Europe and Asia. According to the pandemic risk assessment framework, viruses that exhibit a specific set of traits should be prioritized for surveillance.

The 1C H1N2 virus checks nearly every box. It evades existing immunity in most people, binds to human-type airway receptors, and replicates efficiently in human bronchial cells. It remains stable in airborne droplets under typical humidity conditions and has a low pH of fusion—meaning its surface proteins can undergo the necessary changes for cell entry within the acidic compartments of human airway cells.

In animal studies, it transmitted through the air both to naïve ferrets and to those with prior immunity to the H1N1 strain responsible for the 2009 pandemic. Taken together, these features place 1C H1N2 among the highest-risk flu viruses currently circulating in animals. It’s important to note that meeting the biological criteria for pandemic risk does not guarantee a severe outcome.

The 2009 H1N1 pandemic virus also spread widely and met many of the same risk thresholds, but caused relatively mild illness in most cases. That experience was a reminder that pandemics can vary in severity—and that a virus can be highly transmissible without being highly lethal. The current study does not evaluate how deadly the 1C H1N2 virus might be.

In fact, infections in pigs were mild, and the virus is not classified as a highly pathogenic strain like the H5N1 avian flu currently circulating in U.S. dairy herds and poultry.

Still, the ability to spread efficiently in humans—especially when combined with low levels of immunity—makes early detection and risk assessment essential. The emergence of 1C H1N2 underscores the importance of both structured risk assessment and detailed experimental investigation. In this case, researchers combined serology, molecular virology, and animal models to evaluate a virus that would otherwise remain hidden in the background of routine surveillance.

Their work demonstrates how careful, labor-intensive studies can bring clarity to potential threats before they escalate. While it remains uncertain whether this virus will ever spread widely in humans—or how severe it might be if it does—this study provides a strong scientific basis for keeping it on the radar..