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Researchers at two Canadian universities say the first type of influenza virus people are exposed to in early childhood dictates their ability to fight the flu for the rest of their lives.

They say results of a study suggest exposure to one of the two flu strains that circulate every year – H1N1 or H3N2 – imprints itself on a person’s immunity and disproportionately affects their lifelong response to the flu.

The researchers at McMaster University in Hamilton and the University of Montreal say their findings could allow public health officials to assess who might be at greater risk in any given year, based on their age and the type of viruses that were dominant when they were born.

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Their study is published this week in the journal Clinical Infectious Diseases and based on data from the 2018-19 flu season, which was highly unusual because both strains of influenza A dominated at different times.

Matthew Miller, a co-author on the study and an associate professor at the Michael G. DeGroote Institute for Infectious Disease and the McMaster Immunology Research Centre, said people’s prior immunity to viruses such as the flu could impact their risk of becoming ill during subsequent epidemics and pandemics.

“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 said in a release.

Researchers say a natural experiment occurred last winter in Canada when the dominant H1N1 strain was progressively replaced by H3N2, which accounted for 80 per cent to 90 per cent of flu by March, 2019, allowing them to observe the age-specific incidence of the illness in a season.

“As H3N2 replaced H1N1, the drop in incidence for the 40 to 49 years’ age group noted during the previous 2017-18 season reappeared, while the incidence for older individuals rose markedly with age,” the study says.

The same researchers showed in a study published last year that the elderly in Quebec had a relatively low number of cases of influenza during recent seasons dominated by H1N1, most likely because they’d gained protection from repeated early life exposure to that subtype of viruses that circulated from 1918 to 1956.

The H1N1 strain of influenza A was responsible for the Spanish flu pandemic in 1918, which killed millions of people worldwide. An H1N1 subtype caused the swine flu in 2009, but the connection between the two outbreaks is not clear. The so-called Asian flu of 1957 occurred when H2N2 was dominant, before H3N2 brought along a wave of sickness in 1968.

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