This might explain the data from Canada and Hong Kong (that I have blogged about several times) which showed that receiving flu vaccine in 2008 led to almost double the likelihood of getting sick from swine flu in 2009. In other words, the unvaccinated from the previous flu season fended off swine flu much better than the previous season's vaccinated people did.
This means that catching the garden variety flu could protect you in future from the much-anticipated, threatening Pandemic Flu. And the protection could be expected to last for many decades. Vaccine protection only lasts months, or a year or two. Maybe your body makes lemonade out of flu lemons.
UPDATE: Apparently this article, in French, suggests the same thing:
Geriatr Psychol Neuropsychiatr Vieil. 2011 Mar;9(1):11-9.
Adverse effects of the herd immunity or When childhood vaccination becomes deleterious for the epidemiology of infectious diseases in adults.
Département de réhabilitation et gériatrie, Hôpital des Trois-Chêne, Thônex-Genève, Suisse. firstname.lastname@example.org