"The absence of a sugary viral shield could explain why immune responses to the 1918 influenza virus also work against the 2009 H1N1 (swine flu) pandemic strain.
Researchers have found that the two viruses, although separated in time by nearly a century, are structurally similar in a region that is recognized by the immune system. In seasonal flu viruses, that region — a part of the haemagglutinin protein often used to create flu vaccines — is dotted with sugar molecules; however, the two pandemic flu strains lack this sweet spot. This might explain why seasonal flu vaccines don't protect against swine flu."
Click source for more.