The First Flu You Caught as a Child May Determine Future Immunity

Researchers are still unsure of all the details as to what this may mean going forward, but they may have shed some new light on how we develop long-term immunity against influenza strains. More specifically, they’ve linked the strongest immune responses to the variants we were exposed to as children. 

Different Influenza Strains

Up until 1957, H1N1 was the only known flu virus strain in humans. Since then, researchers have identified several other strains, which they classify by Lineage, Protein, Host, and Origin:

• Lineage - All human influenza viruses fall into Type A, B or (rarely) C. Influenza A viruses are more likely to cause pandemics, whereas Influenza B is less likely to cause severe disease. Humans rarely fall ill with Influenza C.
• Hemagglutinin (HA) - This type of protein occurs on the virus’s outer surface and is responsible for cell entry. In humans, there are two HA groups: Group 1 includes subtypes H1 and H2; the second group includes only H3.
• Neuraminidase (NA) - This protein aids in cell entry and the virus’s exit. N1, N2 and N3 are examples of NA proteins.
• Original host - Influenza A strains often originate from birds or swine, whereas Influenza B strains can only come from and infect humans. 
• Country, year of origin and strain number - These specs help differentiate similar strains that may have the same lineage and HA/NA subtypes.

Putting these bits of info together, the CDC lists the classification for H1N1 strain #35, originating in 1976 in Alberta ducks, as an example: “A(H1N1), A/duck/Alberta/35/76.” 

HA and NA subtypes are likely the most important factors when determining influenza immunity, with our immune responses generally limited to one or the other. For example, our response to an H1N1 strain is dependent on whether we have antibodies against H1or N1; our response to H3N2 would depend on whether we have antibodies against H3 or N2.

How we respond to a given strain might be far more complicated than whether or not our bodies simply recognize these proteins, however.

Creating Long-Lasting Immunity to the Flu

Results of a 22-year study covering 9,541 cases of H1N1 and H3N2 showed the strains we catch as children strengthen our immune responses specifically against that strain and only that strain. Even more, we don’t appear capable of adopting long-term immunity against any of the other flu strains we encounter later in life.

This phenomenon is likely why so few people born before 1957 were susceptible to the 2009 H1N1 pandemic, and why H3N2 has been so devastating to that same age group. Experts are still trying to understand this viral “imprinting,” but it could be one reason why yearly vaccination efforts seem to offer only partial immunity to much of the population.

The Role of Flu Mutation

Another issue is viral mutation, which can slightly alter the way strains evade (and invade) the immune system. All influenza viruses mutate rapidly to stay ahead of host defenses, but some strains, like H3N2, evolve faster than others. More research into how age-specific HA and NA responses persist, despite how these mutations could help formulate better defense strategies in the future.

The flu might still be a step ahead of us, but we’re quickly closing the gap. As experts shed more light on viral mutations and defenses, we can hope to develop better strategies against this yearly invader. Between 250,000 and 500,000 people die each year from the flu worldwide, and we'd really like to get ahead of that curve.

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