The year in which you were born may predict your risk of getting some types of the flu, a new study of people in Asia and the Middle East suggests.

Researchers found that the people in this study who were born in 1968 or later were less susceptible to a certain strain of the flu than those born before 1968, because this older group had been more exposed to a similar strain as children.

In the study, the researchers looked at data from more than 1,400 people, predominantly in Asia and the Middle East, who had been infected at any point in their lives with two strains of the bird flu called H5N1 and H7N9.

The new findings could help researchers predict which age groups are most at-risk for infection and severe illness in future flu pandemics, said the study, published today (Nov. 10) in the journal Science. A "pandemic" means that an illness is very widespread and infects a lot of people. 

"In the past, we always assumed that when pandemic flu viruses emerge from animals, the human population is an immunological blank slate," said lead study author Katelyn Gostic, a graduate student in ecology and evolutionary biology at the University of California, Los Angeles (UCLA). In other words, researchers assumed that everyone's immune system would be defenseless against a new, widespread strain of the flu, she said.  

But the new results suggest the opposite: that a lot of people might actually be immune against a new strain of flu virus from animals if they were exposed to a similar strain as children, Gostic said.

The flu and your age

Scientists had previously noted that a person's age appeared to play a role in determining which virus that individual got during flu outbreaks: While H5N1 typically affected children and young adults rather than older people, the other virus, H7N9, was found predominantly in older adults.

In the new study, the researchers found that the dividing line between the two age groups was 1968, the year of the so-called Hong Kong flu pandemic. 

The virus that caused the Hong Kong flu replaced other flu viruses from a genetically different virus group that had been responsible for most flu cases in the past half century.

The H7N9 strain of the virus, which was shown to mainly infect older adults, is genetically similar to the Hong Kong flu, according to the study. Older adults, born before 1968, were not exposed to this type of virus as children.

Instead, older adults were exposed to the strains of flu that were common before 1968, which are genetically similar to the H5N1 strain. Thus, the older adults in the study were more protected against the H5N1 strain, the study said.

Conversely, people born in 1968 or later may have been exposed to the Hong Kong flu-like H7N9 strain as children. As a result, they were less likely get sick from this strain and more likely to get sick from the H5N1 strain, because they were not exposed to similar strains as kids.

Your first flu

In other words, the type of virus that the people were exposed to during their first-ever flu infection as children determined which type of newbird flu strains they would be immune to in the future.

"Our findings show clearly that this 'childhood imprinting' gives strong protection against severe infection or death from two major strains of" bird flu, James Lloyd-Smith, a professor of ecology and evolutionary biology at UCLA and a co-author of the study, said in a statement.

The new research "is a real step forward for the public health community and those tasked with protecting the population from influenza outbreaks — especially viruses like avian flu that jump from animal reservoirs to humans," said Dr. Michael Grosso, medical director at Northwell Health's Huntington Hospital in Huntington, New York, who was not involved in the study.

Originally published on Live Science.