Not materially different from your immune system killing off the weaker individuals. A few stronger individuals may survive, and then what has your immune system done? Selected for a stronger pathogen.
I remember a paper from a few years ago which concluded that this was basically how we wound up with deadly diseases in the first place -- being the ones that throughout history have managed to be stronger than the host's immune system.
Vaccine simply cuts out the stage where lots and lots of hosts get sick or die.