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.