A warning has emerged that the H9N2 avian influenza virus, which had not drawn much attention, could cause a pandemic. While health authorities worldwide had focused on the deadly H5N1 type, the virus that was taken lightly has instead gained stronger human infectivity.
The journal Nature reported on the 27th that "a research team led by Kelvin To at the University of Hong Kong warned that H9N2, an avian influenza virus previously regarded as relatively 'a mild cold level,' could cause a potential pandemic."
The H9N2 avian influenza virus is named because the hemagglutinin (HA) and neuraminidase (NA) proteins on its surface are type 9 and type 2, respectively. HA acts as the key that allows the virus to attach to human respiratory cells, and NA enables it to break out of cells after replicating.
Avian influenza is usually spread by migratory birds, but in recent years there have been more infections in mammals such as dairy cows and humans. The main culprit is the H5N1 avian influenza virus, whose HA and NA proteins are type 5 and type 1, respectively. H5N1 infects humans and causes severe pneumonia. Since 2020, more than 20 people worldwide have been infected with this virus and have died.
The University of Hong Kong team found that the H9N2 virus, which had received less attention than H5N1, has recently undergone genetic changes that make it bind better to human cells. In experiments, H9N2 viruses collected last year showed far stronger human cell infectivity than the 1999 virus. To date, there have been no reported cases of direct human-to-human transmission of H9N2. However, the researchers said the virus has entered an early stage of gradually adapting to humans.
Michelle Wille, a researcher at the Doherty Institute for Infection and Immunity in Melbourne, Australia, told Nature that "H9N2 infections may in fact be far more common." The virus mostly causes only mild symptoms and is easy to miss in hospital diagnostics, and avian influenza patients tend to be tested mainly for H5N1, she noted.
Wille said, "The problem is that H9N2 is classified as a low-pathogenic virus, so governments are not required to report cases internationally," and warned, "If an animal is infected with several types of avian influenza viruses at the same time, genetic reassortment can occur and a new variant can emerge."
The findings were published on the 16th in the journal Emerging Microbes & Infections and were also presented on the 27th at the Global Pandemic Science Alliance international symposium held in Melbourne, Australia.
References
Emerging Microbes & Infections (2025), DOI: https://doi.org/10.1080/22221751.2025.2576574
Nature (2025), DOI: https://doi.org/10.1038/d41586-025-03519-1