This Flu Strain Has All The Hallmarks Of A Pandemic Threat

A recent study has revealed the presence of an influenza virus in pigs that exhibits characteristics suggesting it could lead to a global pandemic. Known as H1N2, this virus is part of a lineage that originally emerged in birds, adapted to pigs, and has now been identified in humans. Referred to as 1C H1N2, this strain has the potential to infect human airway cells, be transmitted between individuals, and evade existing immunity, placing it at high risk according to a pandemic risk framework outlined in a recent publication in Nature Communications.

Influenza A viruses are not limited to humans but also infect various animal species including birds, pigs, and horses. In pigs, three main subtypes circulate: H1N1, H1N2, and H3N2, which are further categorized into genetic lineages based on their evolutionary history. The 1C lineage, originating in Europe in 1979, emerged when an avian flu virus crossed into pigs, leading to the creation of a new strain with genetic contributions. While the 1C lineage has primarily been confined to swine populations in Europe and Asia, recent detections in humans indicate a concerning trend of increased transmission potential.

Between 2011 and 2024, instances of viruses from the 1C lineage infecting humans have been reported at least 29 times, with a significant rise in cases observed since 2021. While most of these cases seem to be isolated incidents stemming from pigs, the growing frequency raises alarms. Surveillance data also suggests that individuals in close contact with pigs are at higher risk of exposure to these viruses, highlighting the potential for cross-species transmission.

One of the key findings from the study is the limited immunity most individuals have against the 1C H1N2 virus. Blood samples from U.S. adults revealed that only about one-third displayed detectable neutralizing antibodies, primarily among those born before 1950. This lack of protection is attributed to residual immunity from past exposure to 1918-like viruses, with more recent generations showing minimal immune response to 1C H1N2. Furthermore, current seasonal flu vaccines offer little to no cross-protection against this strain.

Laboratory experiments demonstrated several traits of the 1C H1N2 virus that enhance its ability to spread among humans. Similar to the 2009 pandemic H1N1 virus, it exhibits a preference for binding to receptors in the human upper respiratory tract, crucial for human-to-human transmission. The virus replicated effectively in human airway cells in culture and remained stable in aerosol-like droplets across varying humidity levels, mirroring real-world conditions. Additionally, it displayed high neuraminidase activity, a feature associated with increased transmissibility by aiding in cell-to-cell infection.

Transmission experiments involving pigs and ferrets, commonly used as models for studying human influenza, showed successful transmission of the virus from infected pigs to uninfected ferrets without direct contact. Despite the absence of symptoms in ferrets, all developed antibodies upon exposure, indicating transmission. Notably, even ferrets previously immune to the 2009 H1N1 pandemic strain became infected, suggesting existing immunity may not prevent the spread of the 1C virus.

As per the pandemic risk assessment framework, viruses possessing specific traits warrant heightened surveillance. The 1C H1N2 virus meets nearly all criteria, evading existing immunity, binding to human-type airway receptors, and replicating efficiently in human bronchial cells. It remains stable in airborne droplets under typical humidity conditions and has a low pH of fusion, allowing surface proteins to facilitate cell entry within the acidic environment of human airway cells. Animal studies further confirm its ability to transmit through the air to both naïve and previously immune ferrets. Collectively, these characteristics position 1C H1N2 among the most high-risk influenza viruses present in animal populations.

While meeting biological criteria for pandemic risk is concerning, it does not automatically imply a severe outcome. The 2009 H1N1 pandemic virus, which met similar risk thresholds, caused relatively mild illness in most cases. This highlights the variability in pandemic severity and the potential for highly transmissible viruses to not be highly lethal. The study does not assess the virulence of the 1C H1N2 virus, with infections in pigs being mild and the virus not classified as highly pathogenic like the H5N1 avian flu. Nonetheless, the efficient human transmission potential combined with low immunity levels underscores the importance of early detection and risk assessment.

The emergence of 1C H1N2 emphasizes the significance of structured risk assessment and thorough experimental investigation. By utilizing serology, molecular virology, and animal models, researchers were able to shed light on a virus that would otherwise go undetected through routine surveillance. While the extent of its spread in humans and its potential severity remain uncertain, this study provides a solid foundation for monitoring and addressing the risks associated with this virus.