Why your birth year may increase your risk of dying during a flu pandemic

New research suggests people born during a flu pandemic have a higher risk of death during a later flu pandemic.

According to researchers at McMaster University and Université de Montréal, people born during the 1957 H2N2 pandemic, or Asian flu, were at a higher risk of dying during the 2009 H1N1 swine flu pandemic, as well as the resurgent H1N1 outbreak in 2013 and 2014.

And it's not the first time this has happened.

The results were published today in mBio, a journal published by the American Society for Microbiology.

"What we were endeavouring to do in this study was to see whether people who were born during the year of a pandemic were at elevated risk of death during subsequent pandemics that they lived through," said Matthew Miller, senior author of the paper from McMaster University. 

What we see is that people who are exposed to unrelated strains are actually susceptible and that has never been observed before.- Matthew Miller, McMaster University

They first observed the "phenomenon" in cases going back 100 years. 

The team found that people who were born in 1890 experienced the highest risk of death during the 1918 Spanish flu pandemic. That was also the last previous year in which the population experienced a flu pandemic.

Miller says they were interested to see if similar patterns were happening in more modern pandemics.

This led them to discover the elevated risks of death for people during the 2009 Swine flu who were born in 1957 during the H2N2 pandemic. 

Researchers looked at population level data from both the U.S. and Mexico.

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"This basically showed us that this unusual elevated risk of dying as a result of being born during the year of a pandemic is also still happening," said Miller. "This elevated risk, is a risk that spikes higher than what we would predict based on people born in surrounding years."

Another issue the research sheds light on is the notion of immunity.

Miller says while we used to think that even if people were exposed to unrelated strains, they might get some protection or have a net-neutral effect, but what his research is showing is that it actually elevates your risk of death in this case, contrary to the pre-existing paradigm.

"What we see is that people who are exposed to unrelated strains are actually susceptible and that has never been observed before," said Miller.

What to do with the findings

Miller says the research could help during future pandemics.

"The major implication of the study is re-evaluating who's really most at risk when pandemics happen," said Miller.

He says one of the implications of this is the importance of early vaccination in children, to prevent early infections because "it seems like this has a disproportionately problematic effect later in life when pandemics happen", said Miller.

"On average pandemics happen every 25 years so most individuals are exposed to sort of multiple pandemics over the course of their life, it's not a rare occurrence."

Miller says it's important to understand the biological basis in addition to identifying them as potentially high-risk people during a subsequent pandemic.

"If we understand the biological basis of what's going on we can develop therapeutic strategies to potentially help those people in the event they are infected with another pandemic strain," said Miller.

"Our real hope is that these studies will spur more investigation to understand the underlying biology about why this is happening," he said. 

Study raises more questions than answers

Brian Schwartz, Science and Health Protection Public Health Ontario Vice-President, says the study raises key questions.

"What this might do in the next pandemic is it might stimulate us to look at the 2009 birth year and at least look at them more closely with respect to complications while the flu vaccine is being rolled out and making a decision as to whether they might be a higher priority than those born immediately before or after," said Schwartz. 

He says a lot more research needs to be done, especially to look at other jurisdictions to add more validity. 

Ross Upshur, University of Toronto professor at the Dalla Lana Faculty of Public Health says the research has one clear message. 

"They've clearly shown this excess of risk in people born in particular years and that just tells us that there's more to the risk profile of influenza than we previously thought," Upshur said.