As H5 avian influenza viruses continue to spread through birds and mammals worldwide, a pressing question for pandemic planners is how much immunity humans already hold. A study published in October 2026 in the Journal of Virology offers a cautiously encouraging answer. The work was led by Andrés Finzi, professor of microbiology, infectious diseases and immunology at Université de Montréal and researcher at the CRCHUM, with doctoral student Katrina Dionne as first author and collaborators at CHU Sainte-Justine, Université Laval and the University of Ottawa.
The team collected blood from 24 health care workers at the CHUM before and after their annual flu shot, then tested whether their antibodies could recognise a range of influenza strains, including several highly contagious H5 lineages circulating in North America and Asia. Most participants already carried antibodies against these viruses before vaccination, and in some cases those antibodies could neutralise the viruses or eliminate cells displaying viral proteins on their surface.
Seasonal vaccination against H1N1 and H3N2 induced antibodies in some participants that recognised H5Nx viruses, but the researchers observed no marked increase in cross-reactivity after the jab. They suggest this pre-existing immunity reflects each person's accumulated history of past infections and vaccinations. Crucially, it remains unclear whether the seasonal shot protects against H5N1 itself: the authors argue that vaccines specifically targeting avian strains will still be needed, while their findings help define the baseline immunity on which such vaccines would build.