Being older is a good defense against 2009 H1N1 influenza virus

Why is the incidence of infection with 2009 H1N1 influenza highest among 5-24 year olds, and lowest in those over 65 years of age? Were the oldsters previously infected with a related influenza virus, or is there another explanation?

The sera of individuals born in the early part of the 20th century have antibodies that block infection with the 2009 H1N1 virus. We also know that antibodies that prevent infection with recently circulating seasonal H1N1 viruses do not react with pandemic H1N1 strains. These findings may partly explain the lower incidence of influenza this year in individuals greater than 65 years of age (illustrated).

But other factors might also be responsible for safeguarding the older population. Infection of guinea pigs with a 2007 seasonal H1N1 virus confers some protection against infection 3 weeks later with a 2009 H1N1 strain. The animals produced significantly less virus in their respiratory tracts, and transmission of infection to naive animals was impaired. The pre-inoculated animals were also less susceptible to acquiring the 2009 H1N1 virus from another infected guinea pig. Similar results were observed when guinea pigs were first inoculated with a seasonal H3N2 strain and challenged with the pandemic virus. The conclusion is that infection with seasonal H1N1 or H3N2 viruses provides protection against infection with the 2009 H1N1 strain.