In the USA, the CDC (Center for Disease Control) are looking at whether or not flu vaccinations are really effective or not. There is no single flu virus, there are countless versions of it and each time it passes from host to host, it mutates in to something else. By the time vaccines are publically available, the virus is quite different, and varies from region to region.
The flu vaccine gives you a dose of inactivated flu viruses, which then stimulate your body to produce antibodies to fight this virus, but ONLY this virus. To learn more, read this article by Anne Marie Helmenstine, Ph.D.