I do like this kind of movie premises, I found it similar to "The Happening" or "Blindness" and other apocalyptic stories that are not zombies. With these kind of movies I find that the explanation of the events is not necessary really, what's interesting is the situation in which the characters are and how they act. So the strength of the story will be on the characters themselves. Then you can have some good cinematography to back up a creepy atmosphere and the ending to give a twist or shock. Here I found pretty decent characters, good cinematography and an ok ending, none are great, but it does the job of keeping me interested, not annoy me with extremely stupid behaviours (there was a scene which I found quite silly near the ending, and was quite important, with the main characters, but...) and not disappointing me with an awful ending. Still, it fell a bit bland in the end, and as I said, everything is ok, but nothing exceptional. It definitely had potential for a better movie.