From what I understand, the vaccine is injecting the flu into your system and having all those nifty white blood cells attack it. Doing it it is programed to be able to kill off the raw flu germ when you are exposed causing you to have a more moderate reaction to it, if any at all. It's the real flu that will kill you, not the vaccine.
I am a very strong advocate for vaccines and will take anyone the comes along and has shown some sign of being helpful. I remember two things about my childhood the grouping of Mumps, Measles, Whooping Cough and Chicken Pox and how sick I felt when I got them and also the biggy Polio. I didn't have Polio because it was terrifying seeing all those kids in Iron Lungs so when someone said they had invented a vaccine we didn't walk, we ran to get it. They were giving them in fire stations and town halls all over the country and stopped that evil thing in it's tracks. Because of those things happening they didn't happen to my children. I cared enough to not have them experience the same thing I did. I still had one child that grew up to not believe in vaccines but fortunately she had her children vaccinated when it was legally and socially required. Why? Because she never got those things mostly because her mother and I knew the value whereas she never experienced those illnesses so therefore she has no reason to fear them.