From what most of the vaccines seem to be is a marker (protein used for such) on am entity they want destroyed. Tells your immune system if you see guys with a red x on their shirt, start taking them out. If you don’t have the vaccine they enter and no one knows the red x is bad. They start to run rampant and doing bad things until something done bad enough is recognized and they say, hey shoot the ones with the red x. In both scenarios the red X’s infiltrated your body, in 1, your body is able to start defending itself sooner and it lessens the damages caused vs what may have occurred if it didn’t know to start attacking sooner. (That does not mean someone’s immune system couldn’t be worse than someone else’s and still react slower… just faster than it would have otherwise)