A coma is a profound and extended length of unconsciousness. I mean really profound and extended. I’m talking days, weeks, months even years of extended, profound lack of consciousness.
In the zombie fiction prior to the First Outbreak, the story’s protagonist often was a recently awakened coma patient. This explained their complete and utter lack knowledge about the situation fate thrust them into, and thus heightened the drama of the work. Unfortunately, overuse had turned the premise into a cliché.