omg
There are dozens of different strains of the influenza virus, the H and N that you hear about (e.g, H1N1) refer to antigens (proteins on the surface of the virus which cause generation of antibodies, the things that eventually kill the virus).
The vaccine contains the specific combination of these antigens for the type of virus which is active in that particular year. Each year, there is genetic drifts or shifts in the virus meaning that the vaccine you got in 2010 will NOT protect against all strains that are virulent in 2011. Influenza is not just "one single virus", it wears a range of different "jackets and trousers" (hemagglutinin and neuraminidase , or 'H' and 'N') which is how it manages to evade the immune system.
It
IS actually possible to have more than one strain of the flu at one time, more likely if both the H and N are different between the 2 viruses. Also, the "swine flu" is JUST another strain of influenza. H1N1 - it's not an alien DNA exactly, it just so happens to have jumped the species barrier, much like H2N2/H3N2 etc....(avian) and most types of influenza at some point. Seems to be the way the media have portrayed H1N1 as "OMG BIG EVIL SWINE FLU!" that's caused people to panic about it and see it as some kind of massive threatening evil form of the flu rather than......just another strain :/
The hospital where I work - a particular ward in which 3 patients had been diagnosied with swine flu - the consultant had decided just to test every patient on the ward regardless of symptoms - and it was found that another 4 patients had the virus, but only had extremely mild common cold-type symptoms!
Anyway, sorry for rambling, I've went on a bit to much and went off on a tangent.........shoot! I have a degree in toxicology & immunology so virus or bacteria talk makes me quite excited.......
but it might help someone to make a more informed decision regarding the vaccine :S possibly anyway