Volunteers in the United Kingdom have received doses of an experimental vaccine designed to protect against H5N1, a bird flu strain that has caused widespread infection in bird populations and some mammals globally. The UK Health Security Agency describes the current human health threat as low, with documented cases primarily resulting from direct exposure to infected animals.
The trial is recruiting 4,000 participants, with three-quarters enrolled at 26 sites across England and Scotland and the remainder participating in the United States. The study targets individuals working in poultry industries and people aged 65 and older, the two demographic groups considered at highest risk. The vaccine employs mRNA technology, the same platform used in current Covid vaccines, which researchers say allows for rapid development and large-scale production capacity should a pandemic emerge. Since 2024, there have been 116 confirmed human cases worldwide, nearly all associated with close contact with infected animals.
Dr Rebecca Clark, the trial’s national coordinating investigator, emphasized that while human-to-human transmission has not yet occurred readily, it must be treated as a genuine possibility. The research will evaluate vaccine safety and its ability to generate immune response, with potential licensing for use if circumstances warrant. Manufacturing capacity exists through Moderna’s facility in Harwell, Oxfordshire, which currently produces Covid vaccines for the UK and can manufacture 100 million annual doses, with capacity to increase to 250 million doses during a pandemic.
The mRNA approach offers advantages over traditional egg-based flu vaccine production methods, which face complications when virulent avian strains kill the eggs used in manufacturing. The trial received £40 million in funding from CEPI, the Coalition for Epidemic Preparedness Innovations, after the US government reduced mRNA vaccine funding in August 2025. As part of the funding agreement, Moderna has committed to ensuring fast and affordable vaccine supplies to low and middle-income countries in any future pandemic, addressing concerns about vaccine distribution inequality experienced during the Covid pandemic.
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