A study conducted at Université de Montréal’s affiliated hospital research centre (the CRCHUM) sheds new light on the immune response to avian influenza viruses. Most participants already had antibodies capable of recognizing several of these viruses before receiving the seasonal influenza vaccine.

To better understand the extent of immunity to avian influenza viruses, Katrina Dionne, a Université de Montréal student and doctoral candidate at the CRCHUM under the supervision of Andrés Finzi, a researcher in the Immunopathology Research Theme, analyzed blood samples collected before and after seasonal influenza vaccination from participants in the cohort of CHUM health care workers. She then assessed their antibodies’ ability to recognize different influenza virus strains, including several H5 avian influenza viruses from North America and Asia.

What pre-existing immunity reveals

The results of the study, published in the Journal of Virology, showed that before vaccination, most participants already had antibodies capable of recognizing several avian influenza viruses. In some cases, these antibodies were also able to neutralize the viruses and promote the immune-mediated elimination of cells displaying viral proteins on their surface.

“What particularly surprised us was finding that this cross-reactivity was already present before vaccination in several participants. This raises important questions about how our previous exposure to influenza viruses may influence our response to new avian influenza viruses,” explains Katrina Dionne, first author of the study and a recipient of a Canadian Institutes of Health Research scholarship.

According to Andrés Finzi and Katrina Dionne, this pre-existing immunity may result from previous exposure to different influenza viruses or from vaccinations received over the years. The findings suggest that each person’s immune history helps shape their ability to recognize new, related viruses.

Better understanding the response to emerging viruses

The study also shows that, in some people, seasonal vaccination against the H1N1 and H3N2 strains induces antibodies that are capable of recognizing H5Nx viruses. However, the researchers did not observe a marked increase in this cross-reactivity following vaccination.

These findings therefore do not establish that the seasonal influenza vaccine protects against H5N1 or other emerging avian influenza viruses. Rather, they underscore the importance of developing vaccines specifically designed to target these viruses in the event of future outbreaks.

“This study gives us a better understanding of the extent of immunity already present in the population against certain avian influenza viruses. This knowledge could contribute to the development of vaccination strategies offering broader protection against future emerging influenza viruses,” says Andrés Finzi.

Conducted in collaboration with researchers from the CHU Sainte-Justine, Université Laval, and the University of Ottawa, the study, Pre-existing cross-subtype serological responses against H5Nx in health care workers, was published in the Journal of Virology on October 7, 2026.