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A woman is silhouetted as she uses a tissue to blow her nose in Toronto on Friday, November 21, 2014. (Nathan Denette / THE CANADIAN PRESS)

The first type of flu virus we are exposed to in early childhood dictates our ability to fight the flu for the rest of our lives, according to a new Canadian study.

A team of infectious disease researchers at McMaster University in Ontario and University of Montreal in Quebec have provided new evidence to support the phenomenon known as “antigenic imprinting.”

The research suggests that early exposure to one of the two flu strains that circulate every year, H1N1 or H3N2, imprints itself on the immune system and disproportionately affects the body’s lifelong response to the flu.

And researchers say those findings can help scientists better understand outbreaks like the one involving the novel coronavirus.

“People’s prior immunity to viruses like flu, or even coronavirus, can have a tremendous impact on their risk of becoming ill during subsequent epidemics and pandemics,” Matthew Miller, a study co-author and an associate professor at the McMaster Immunology Research Centre, said in a news release. 

“Understanding how their prior immunity either leaves them protected or susceptible is really important for helping us to identify the populations who are most at risk during seasonal epidemics and new outbreaks.”

The researchers found that people who were born when H1N1 was dominant have a much lower susceptibility to flu during seasons dominated by that strain of the virus.

In turn, those born in a H3N2 year are less vulnerable to flu during seasons dominated by H3N2.

Researchers analyzed data from the 2018-19 flu season, which was “highly unusual” because both strains dominated at different times. Typically, only one strain dominates each flu season and will account for almost all cases.

Alain Gagnon, professor of demography at the University of Montreal and lead author of the study, said the new research goes “much further in support of antigenic imprinting.”

“We already knew from our previous studies that susceptibility to specific influenza subtypes could be associated with year of birth,” Gagnon said in the press release.

“Instead of just showing how specific age patterns are associated with one subtype or the other during a single influenza season, we took advantage of a unique ‘natural experiment’ to show how the change in subtype dominance during one season appears to lead, practically in real time, to a change in susceptibility by age.”

The academics behind the study said the new evidence could help public health authorities to assess who might be at greater risk in a given year, based on their age and what viruses were dominant at the time of their birth.

Health Canada estimates that the flu is responsible for around 12,200 hospitalizations and 3,500 deaths every year.

The findings were published this week in the journal Clinical Infectious Diseases.