H9N2 Avian Flu Quietly Evolves Toward Humans, Scientists Warn

As the world focuses on the deadly H5N1 bird flu, another avian influenza virus—H9N2—is quietly gaining new abilities. Once considered a mild strain limited to birds, H9N2 is showing signs of adapting more efficiently to humans, according to new research.

Although H9N2 is the second most common avian influenza strain infecting humans, with 173 confirmed cases since 2015, it has long remained in H5N1's shadow. Experts believe that many mild or asymptomatic infections may have gone undetected, meaning the virus's true spread could be far greater than reported.

Laboratory data have raised concerns: compared with strains from two decades ago, recent H9N2 samples collected in 2024 infect human cells far more effectively and show a stronger binding affinity for human receptors. This suggests the virus is evolving toward better compatibility with the human body.

However, several biological barriers still prevent H9N2 from spreading efficiently between people. To achieve sustained human-to-human transmission, the virus would need to prioritize human-type cell receptors and adapt to the body's internal temperature and chemical environment. If those hurdles are overcome, experts warn, the risk could increase dramatically.

Even more worrisome, H9N2's genetic material has already been detected in other flu viruses that have infected humans, suggesting it can recombine with other strains to produce new, more transmissible variants. Such reassortment could occur in animals—or even humans—co-infected with multiple influenza viruses.

Currently, global monitoring systems have major blind spots. “Low-pathogenic” strains like H9N2 are not always subject to mandatory reporting, allowing them to circulate largely unnoticed. Scientists are urging enhanced surveillance of mammals that come into close contact with domestic and wild birds, as these animals could serve as training grounds for the virus's next evolutionary leap.