The Year You Were Born Determines Which Strains Of Flu You're Vulnerable To

The H5N1 virus, shown here, mainly affects people born after 1968, and we now know why. Older people are more vulnerable to H7N9. Cynthia Goldsmith

It sounds like astrology, but one of the world's most prestigious scientific journals has reported that the date of your birth determines which sorts of influenza you are most at risk of catching. The finding resolves a long-standing puzzle about the way flu outbreaks spread, and could help control future epidemics.

Flu infections are usually most dangerous for the elderly, the very young, or people with compromised immune systems. However, some outbreaks don't obey the rule. The H5N1 strain, for example, mostly affects the young, while H7N9 is a terror in retirement homes. The devastating effects of the 1918 to 1919 Spanish flu pandemic reflected the fact that most of those who died were young and middle-aged adults.

Explaining this troubles virologists racing to ready the world for what they see as an inevitable future outbreak. In the journal Science, a team led by Professor James Lloyd-Smith of UCLA have presented a possible answer.

The authors propose that there are similarities between apparently different strains of the virus, and that childhood infection by strain provides partial protection against strains that were previously seen as unrelated. People of a particular generation are likely to have encountered certain strains, and when they are exposed to a different, but somewhat related, virus, they are less likely to get very sick.

This sort of cross-protection is widely accepted, but Lloyd-Smith proposes that it applies to far more flu varieties than previously recognized, with all known versions of the virus being parts of groups where they protect against other members of that group.

The evidence comes in the form of the discovery that some people are immune to certain flu viruses that exist in animals, but have not crossed over to the human population. This immunity could not come from being previously infected by those exact strains, nor from vaccination.

By investigating who has protection against particular strains, Lloyd-Smith and his colleagues were able to group the viruses' varieties together. They found people born in particular years, who were likely to be caught up with particular flu outbreaks while very young, carry common resistance. They refer to this as “childhood imprinting”.

"Our findings show clearly that this 'childhood imprinting' gives strong protection against severe infection or death from two major strains of avian influenza," Lloyd-Smith said in a statement. Which strain people are protected against largely depends on when they were born, with 1968 representing the turning point. Americans born before that date more likely to have protection against H7N9 than H5N1, those born after it are relatively immune to H5N1.

This sort of immunity will not always prevent people from getting sick, but will reduce the severity of their symptoms if they do. In 1918 older people who would normally have been the most vulnerable to the epidemic probably carried some protection from a related strain that had disappeared before those in their teens or twenties were born.

The findings should be of use to public health officials in the event of a pandemic, allowing them to work out who is most likely to need protection. It may also assist in the design of a universal flu vaccine.

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