Does the vaccine against 2009-2010 seasonal influenza also provide cross-protection against the A/H1N1 “swine” pandemic flu virus?

No, cross-protection is not provided between the 2009-2010 seasonal influenza and A/H1N1 pandemic vaccine preparations. The pandemic “swine flu” virus is very different genetically from seasonal types. The latter constantly change by a process called “antigenic drift.” This alteration results from minor point mutations in the viral genetic material during replication in host cells. As a consequence these viruses can routinely change from one season to the next, or in some instances, even within a single flu season.

In contrast, pandemic influenza strains arise from a different mechanism involving genetic recombination between multiple influenza A virus subtypes. This process, termed “antigenic shift,” occurs from influenza viruses from different species co-infecting the same animal host (i.e. the pig), whose cells have receptors for human, bird, swine flu viruses. Whole segments of viral nucleic acid can be exchanged in creating progeny viruses. The resultant viruses contain segments of RNA from each of the original viruses, are typically are completely different in their antigenic makeup from any previously known strain, and often are more virulent than seasonal flu viruses.

When new influenza A viruses are introduced into the human population, infected persons have little or no protective immunity from previous exposure to other influenza viruses. In the worst case scenario, when these strains adapt to efficient human-to-human transmission, the potential for a widespread pandemic arises with a high mortality rate.