Interesting April theory by Kim Newman
(Spoilers)
This was an interesting thought by Kim Newman in his review of the movie:
“… a few hints (a period photograph, a shot of April with blood on her mouth, the immunity to a blood-borne disease) suggest the girl may be an immortal (a vampire?); the old photograph is provisionally explained away as her grandmother and a substitute backstory involving neglectful, perhaps-abusive grandparents is flashed up without quite being confirmed.”
https://johnnyalucard.com/2020/04/07/film-review-the-dead-outside-2008/
I think a mashup of something like 28 Days Later and Let the Right One In could be an interesting idea. It would have been interesting to see this movie go down that road.