That's pretty much a question of what the divide between zombies and vampires really is. They are, in essence, very similar, especially compiling variants of t

That's pretty much a question of what the divide between zombies and vampires really is. They are, in essence, very similar, especially compiling variants of the myths. They are undead in some way feeding off of humans in a manner that, some percentage of the time or by some means, turns the human into one of them. Pre-Dracula vampires are generally hideous in manners that could be confused with some versions of zombies. The book is pretty explicit about them being vampires including distinctly vampiric traits (nocturnal, traditional vampire weaknesses, killing them with stakes through the heart. The film of the same title, however, rides the line pretty close, coming closer to vampires with the original ending. Their weakness to sunlight and their intelligence are clearly more vampiric, but it doesn't really do much more than that. Either way, though, Matheson's influence on zombie fiction certainly makes the vampires look more zombie-ish from a modern perspective and can probably be uncontroversially explained at least as a stepping stone from between vampires and zombies.