Why Does A/H1N1 Swine Flu Mainly Affect Young Adults?

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Influenza, Killer of the Old and Young

Deaths from seasonal human influenza primarily affect young children and the elderly. In a typical season, a graph of influenza deaths by age group has a U shape, with peaks for the very young and the very old and a valley for adolescents to middle-aged adults.

The 1918 “Spanish flu” pandemic was different. It had a third peak for relatively young adults between the ages of 25 and 34. A graph of 1918 influenza deaths by age group has a W shape, with peaks for very young children, young adults, and the elderly (Taubenberger & Morens 2006).

Swine Flu, a Disease of Young Adults

When A/H1N1 “swine flu” broke out in North America and then the world in 2009, initial reports suggested it may have been following the pattern of the 1918 flu. In its first update on 2009 A/H1N1 swine flu, dated April 24, the World Health Organization reported:

“The majority of these cases have occurred in otherwise healthy young adults. Influenza normally affects the very young and the very old, but these age groups have not been heavily affected in Mexico.”

It further noted that the fact that young adults were heavily affected was “of high concern.”

The A/H1N1 swine flu has turned out to be far less deadly than was initially feared. Yet as the pandemic has progressed, a peculiar pattern has emerged: the elderly are not catching it, by and large. Why do older people seem to be immune to swine flu?

Why are the Elderly Immune?

A possible answer to this puzzling question was published as a report in the journal Science in May 2009: The elderly may have already been exposed!

To understand how this is possible, it is necessary to know the history of the H1 component of influenza A. H1 is a subtype of the virus protein hemagglutinin. The H1 in both swine flu and human flu has a common ancestor.

Between the 1918 and 1957 influenza pandemics, H1 circulated in humans, evolving continuously. H1 then became inactive, replaced by other subtypes of hemagglutinin such as H3. H1 in humans reappeared in 1977 and has been a dominant subtype of human flu ever since. It has evolved significantly from 1977 to the present. One could call this version “New H1.”

In pigs, meanwhile, the H1 has evolved very little. Swine flu H1 has been very similar to the “original” 1918 and 1930 versions of H1. One could call this version “Old H1.” To the immune system, Old H1 is very different from New H1.

It may be speculated that most people born before 1957 were exposed to human H1 influenza, which at that time was similar to Old H1. Their immune systems produced antibodies to Old H1. Since Old H1 has not been seen in humans in many years, younger people do not have antibodies to Old H1.

Meanwhile, the H1 in 2009 A/H1N1 swine flu — as in all H1 swine flu — is very close to Old H1. Since most older people probably have antibodies to Old H1, they are immune to this new swine flu. Younger people are not so lucky, and this is why swine flu mainly affects younger adults.

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