When flu strikes, why are some family members reduced to shivering wrecks under their duvets, while others get off with little more than a snuffle?

当流感来袭时，为什么有些人蜷缩在羽绒被下瑟瑟发抖，而有的人最多就是有点鼻塞呢？

Scientists now have an answer, showing that the generation you belong to - and even the year of your birth - predicts how vulnerable you will be to a given strain of seasonal virus.

The flu virus a person first encounters as a child, they found, leaves a permanent "imprint" on the immune system, giving them robust protection against similar strains and much weaker protection against less closely related varieties of the illness.

Michael Worobey, head Ecology and Evolutionary Biology at the University of Arizona and a senior author of the study, said: "It's not the age, it's the birth year that matters."

In future, seasonal vaccines could be targeted at people of particular ages who are most likely to benefit, and in pandemics when medications are scarce, vulnerable age ranges could be prioritised for protective measures.

"It's breaking new ground for flu, where predictions are really hard," said Worobey. "For any given potential pandemic virus, we can actually now say ... this is the age group that you can expect is going to end up in hospital dying and this is the age group who will be protected."

The age effect is seen because influenza A viruses - the kind considered most likely to cause pandemics - have evolved into two major branches known as type 1 and type 2 flus.

Up until 1968 all viruses in circulation belonged to the type 1 branch; between 1968 and 1979, type 2 dominated. Since then, strains belonging to both branches have been in circulation simultaneously, but with one type tending to dominate each year. "It's like an oak tree that has a trunk that splits into two major branches and 1968 becomes this really clear dividing line," said Worobey.

Using vast databases of historical epidemiological data, the scientists tracked the susceptibility of each birth year from 1918 to the present to the different flus in circulation during their lifetime.

The findings, published in the journal Science, showed that the strains in circulation early in life - most people have had flu by the age of five - have a profound impact on which types of flu they would be more sensitive to in the future.