Birth year dictates which flu strains make you sick

A person’s year of birth seems to affect the immune system’s ability to protect against different influenza strains, according to a study published in Science.

The study, led by Katelyn Gostic, a biologist at the University of California, Los Angeles, found that lifelong protection against either the H5N1 or H7N9 variants of the Influenza A virus depended on which one caused a person’s first infection.

Both are types of “avian flu” – viruses that crossed the species barrier from birds to humans – but H7N9 belongs to a variant that did so much more recently than H5N1.

Gostic and her team investigated data from all known human cases of both types of virus.

They discovered that people born before the Hong Kong flu pandemic of 1968 – in which H7N9’s ancestor, H3N2, first emerged – were largely protected from the older-type H5N1, but were vulnerable to severe illness from H7N9.

Those born after 1968 showed the opposite responses.

This explains a pattern that has puzzled scientists since 2013, when H7N9 itself was first detected: flu viruses from the H5N1 “family” disproportionately affect children and young adults, while those more closely related to H7N9 are more likely to infect older adults.

According to Gostic and her team, the phenomenon can be attributed to a sort of “immunological imprinting” that occurs when people are first exposed to the flu during childhood.

They note that during this event, our immune system creates antibodies that target haemagglutinin, a lollipop-shaped protein found on the surface of the virus.

There are 18 known haemagglutinin subtypes belonging to two main groups, which study co-author Arizona University virologist Michael Worobey calls “blue” and “orange”.

The study found that the “blue” group, comprising subtypes descending from earlier H1 and H2 flu pandemics, conferred protection against H5N1 viruses.

The post-1968 “orange” group, stemming from the emergence of H3 and subsequent variants, protected against H7N9.

Gostic and her team’s findings can be used to quantitatively assess the risks of future flu pandemics based on demographic information already collected by governments and health organisations around the world.

“These findings challenge the current paradigm, where the entire population would be immunologically defenceless in a pandemic caused by a novel influenza virus,” Gostic says.

“Our results suggest it should be possible to forecast age distributions of severe infection in future pandemics, and to predict the potential for novel influenza viruses from different genetic groups to cause major outbreaks in the human population.”