The findings, published in the journal Clinical Infectious Diseases, provide compelling new evidence to support the phenomenon known as 'antigenic imprinting', which suggests that early exposure to one of the two flu strains that circulate every year imprints itself on our immunity and disproportionately affects the body's lifelong response to the flu.

 

"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," said study researcher Matthew Miller from McMaster University in Canada.

 

"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," he added.

 

For the findings, researchers collected and analysed data from the 2018-19 flu season, which was highly unusual because both strains of influenza A dominated at different periods of time.

 

Typically, only one strain dominates each flu season and will account for almost all cases, said the researchers.

 

The researchers found that people who were born when H1N1 was dominant have a much lower susceptibility to influenza during seasons dominated by that virus than during seasons dominated by H3N2. 

 

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

 

"We already knew from our previous studies that susceptibility to specific influenza subtypes could be associated with year of birth. This new study goes much further in support of antigenic imprinting," said study lead author Alain Gagnon from the University of Montreal.

 

"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," Gagnon explained.

 

Researchers hope to further explore transmission dynamics by analysing how viruses spread within households, where exposure is high and prolonged.