The virus has now been detected on six continents, including areas like Antarctica that had never reported bird flu before
New concerns are emerging over the deadly H5N1 bird flu, which scientists say is mutating faster than ever and entering an unpredictable new phase.
A global analysis uploaded to preprint biology server bioRxiV on November 24 by influenza specialists working with the World Health Organization, the World Organisation for Animal Health and the UN’s Food and Agriculture Organization warns that the virus’ rapid evolution and increasing ability to infect mammals could significantly raise future risks to humans.
According to the H5 Evolution Working Group, which reviewed more than 18,000 samples collected between 2015 and 2024, today’s H5N1 is far removed from the strain that circulated in the mid-2000s.
The virus is now more diverse, having now been detected on six continents, including areas like Antarctica that had never reported bird flu before.
Rather than a single strain circulating globally, H5N1 has split into numerous genetically distinct groups, each evolving in its own direction.
Scientists liken the virus to a rapidly expanding “family tree.” Over the past decade, some branches have evolved so quickly that the virus’ official classification system had to be revised.
Two major branches have now split into 12 sub-branches, showing how much H5N1 has diverged from earlier forms.
One longstanding branch, clade 2.3.2.1c, has been circulating in Asia for years and has caused human infections in Cambodia, Vietnam, and Nepal. It has since fragmented into several new offshoots, including one associated with recent cases in Cambodia.
The country has reported 18 cases this year, nine of them fatal. The most recent death on November 16 involved a 22-year-old man in Phnom Penh.
The rise in severe cases shows that H5N1 is adapting in environments where humans and animals closely interact. This could mean that older poultry vaccines may no longer offer good protection against the viruses now in circulation.
However, scientists are even more concerned about clade 2.3.4.4, which has become a global driver of transmission. This group has spread quickly through migrating birds and mixed with other flu viruses, creating new genetic combinations. One sub-group, 2.3.4.4b, has killed many wild birds and infected an unusually high number of mammals.
The spillover into mammals is particularly troubling. H5N1 has infected mink, foxes, seals, sea lions, raccoons, cats and even dairy cattle in the US.
Since bird flu viruses typically struggle to infect mammals, repeated cross-species transmission suggests the virus is learning how to survive in new hosts. This could make human transmission easier.
Nonetheless, human infections remain rare and there is no sign of sustained person-to-person spread. Most cases involve close contact with sick birds or contaminated settings.
Still, the virus’ growing diversity makes vaccine planning more difficult. Many existing “candidate vaccine viruses” no longer match well with the strains circulating today. Updating them takes time while the virus continues to evolve.
Surveillance also varies widely by region. Limited testing in parts of Africa and Asia means outbreaks in wild birds or poultry may go unnoticed, giving the virus more room to change.
For now, scientists say the overall risk to the public is low. However, H5N1 is no longer the slow-moving virus seen in earlier outbreaks. It is evolving rapidly, spreading widely in birds, repeatedly infecting mammals and generating more genetic diversity than ever before.
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