Canadian researchers found that most of 24 healthcare workers already had antibodies able to recognise H5 bird flu viruses before their annual flu shot, though the study does not show that seasonal vaccination protects against bird flu.
Many people may already carry some immunity to bird flu viruses, Canadian virologists have found after analysing blood samples from healthcare workers before and after their annual flu shot. The samples came from 24 workers at the Université de Montréal teaching hospital, the CHUM.
Before vaccination, most participants already had antibodies against the viruses tested, the researchers found. In some cases those antibodies were able to neutralise the viruses and eliminate cells displaying viral proteins on their surface. The viruses included several highly contagious and widespread H5 varieties found in North America and Asia.
Seasonal vaccination against the H1N1 and H3N2 strains induced antibodies capable of recognising H5Nx viruses in some people, but the researchers did not observe a marked increase in this cross-reactivity after vaccination. First author Katrina Dionne, a doctoral student supervised by immunologist Andrés Finzi, said the pre-vaccination cross-reactivity was surprising and raises questions about how previous exposure to influenza viruses shapes responses to new avian viruses.
The finding is published in the Journal of Virology, and its limits matter: the study involved only 24 healthcare workers, so it shows that pre-existing cross-reactive antibodies exist in this group, not how common they are in the wider population. The researchers are explicit that it is unclear whether the seasonal flu shot protects against H5N1 or other emerging avian influenza viruses, and they say vaccines specifically targeting those viruses will be needed if future outbreaks occur.
The work could help shape broader flu vaccine strategies, Finzi said, by clarifying how much immunity is already present against certain avian viruses. That knowledge matters for planning, but it does not change current advice: the study does not establish protection against bird flu, and any future vaccine would need its own testing before it could be relied on.