The flu vaccine was prepared every year in hen's eggs - each dose had to use one egg, which was one of the reasons it was hard to produce large quantities quickly.
They injected 'it' into the egg, then some time later (several weeks, I think) they harvested the vaccine from the egg. I'm not sure what they did with the eggs - maybe they had a big fry-up every week! :))
I believe they have a quicker method now, more technical and controlled, but I don't know much about it. I expect they miss the Friday fry-ups...
microorganism is killed or attenuated and injected into the human body then it is attacked by white blood cells and can be overcome So that in the second infection antibodies recognize it easily (my knowledge )