H5N1 clade 2.3.4.4b spreads worldwide, reaching mammals and humans
The story begins with a genetic exchange. In 2020, an influenza virus already circulating among bird populations swapped segments of its RNA with other flu strains.
This reassortment produced the clade known as 2.3.4.4b of H5N1. Since that moment the virus has travelled across Europe, Africa and Asia, eventually appearing on every continent except Australia. Its presence has been documented in poultry, wild birds, a range of mammals and, increasingly, in people. As of 15 February 2025, outbreaks have been recorded on all continents bar one.
The pathogen now occupies multiple ecological niches, moving from birds to mammals and humans, a pattern that avian influenza rarely exhibits on such a scale. The emergence of this particular strain is not an isolated incident.
It is the product of reassortment, the process whereby influenza viruses trade genetic material much like trading cards. H5‑2.3.4.4b viruses encountered other avian flu types, exchanged gene segments and gave rise to a new pathogen. Earlier warning signs existed.
Between 2018 and 2020, H5N6 and H5N8 viruses that carried the same H5‑2.3.4.4b hemagglutinin gene were already causing problems worldwide. The 2020 appearance of H5N1 bearing that gene therefore represented a continuation of a trend that had been developing for several years.
Wild aquatic birds serve as the natural reservoir for influenza A. They harbor the virus without showing illness. Transmission occurs through feces, contaminated environments and when one bird consumes another.
These pathways enable the virus to jump to new hosts and to keep moving across geography. As the virus spreads, it continues to encounter local influenza strains in new regions.
Each encounter offers another opportunity for genetic swapping. Every such swap is a roll of the dice; the resulting variant might transmit more readily, infect additional species or cause more severe disease. The global expansion since 2020 has been relentless.
First identified in Europe during the autumn of that year, the virus then moved into Africa and Asia. It ignored continental boundaries and, importantly, species boundaries.
Mammals began to appear in case reports, followed by human infections. Public health agencies have monitored the situation for years. Their concern extends beyond avian losses; they worry about a virus capable of killing millions of poultry acquiring the ability to move efficiently through mammals, and ultimately reaching a highly mobile host—humans.
The current episode fits within a broader pattern of rising frequency and magnitude of avian influenza outbreaks. The H5N1 strain bearing the 2.3.4.4b gene is the latest iteration, but further variants are expected as the virus continues to reassort and adapt.
Surveillance and monitoring remain the only effective tools. By tracking where the virus appears, which organisms it infects and how its genome changes, scientists can anticipate the next evolutionary step. Without such data, any response would be blind. The virus shows no regard for political borders or taxonomic categories; it seeks only new hosts.
With outbreaks now reported on every continent except Australia, the pool of unexploited territories is shrinking.
