Influenza is still a major global cause of death, and resulted in 80,000 deaths in the United States during the 2017-2018 season.
Immunological imprinting may be the key to understanding why the same strain of the flu virus affects people with various degrees of severity, according to a team of researchers.
H1N1 and H3N2 are the 2 strains responsible for seasonal flu outbreaks over the past several decades. H1N1 is most likely to affect young and middle-aged adults, while H3N2 causes the majority of severe cases in high-risk elderly people, and the majority of flu-related deaths.1
Influenza is still a major global cause of death, and resulted in 80,000 deaths in the United States during the 2017-2018 season.1
Immunological imprinting is the idea that past exposure to the flu virus determines a person’s future response to infections, including how well their immune systems are able to fight off the virus.1
The researchers analyzed health records in order to understand whether immunological imprinting could explain people’s response to flu strains already circulating, and to what extent it could account for observed discrepancies in how severely the seasonal flu affect people in different age groups.1
The health record analysis revealed several patterns, most notably that people first exposed to H1N1 during childhood were less likely to be hospitalized if they contracted H1N1 again in later life, compared with patients who were first exposed to H3N2. Similarly, people first exposed to H3N2 received extra protection against that strain later in life.1
For example, patients who had their first case of the flu as children in 1955, when H1N1 was circulating but H3N2 was not, were more likely to be hospitalized with H3N2 in the 2018-2019 season, when both strains were circulating.2
The investigators also noted that patients whose first childhood exposure was to H2N2, a close cousin of H1N1, did not have a protective advantage against H1N1 later in life. This discovery is contradictory to earlier work which showed that exposure to 1 strain can sometimes grant protection against the other.1
“This is perplexing because our research on bird flu shows that deep in our immune memory, we have some ability to recognize and defend against the distantly related, genetic third cousins of the strains we saw as children,” said lead author Katelyn Gostic, PhD.2
While there is still more work to be done to understand the lack of transferable immune response, the researchers hope their findings could help predict which age groups might be most severely infected during future flu seasons.2