Were you born in an H1N1 flu year or an H3N2? Why this matters

The first type of influenza virus you're exposed to may set your lifetime ability to fight the flu.

Researchers with McMaster University and University of Montreal found that being born in an H1N1 year or an H3N2 year matters. Following a phenomenon known as antigenic imprinting, the study revealed that early exposure to one of these two flu strains permanently affects your immunity.

Knowing who is at a higher risk each year could help tailor pandemic and epidemic planning, the researchers say.

"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 co-author Matthew Miller, an associate professor at McMaster Immunology Research Center.

Antigenic imprinting

"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," continued Miller.

Researchers analysed data from the 2018-2019 flu season. Unlike typical flu seasons where one flu strains dominates, both strains of influenza A dominated at different times.

"We already knew from our previous studies that susceptibility to specific influenza subtypes could be associated with year of birth," said lead author Alain Gagnon, a professor of demography at University of Montreal.

"This new study goes much further in support of antigenic imprinting," Gagnon added. "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."

In the United States, flu has led to between 140 000 and 810 000 hospitalisations and between 12 000 and 61 000 deaths annually since 2010, the US Centers for Disease Control and Prevention estimates.

The study was published recently in the journal Clinical Infectious Diseases.

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