ShockTillYouDrop.com have just posted an interview with Daybreakers directors Peter and Michael Spierig. There is no mention of Isabel, but they do talk about the possibility of a sequel:

Question: Have you already decided what happens next?

Michael: Absolutely, we have. We don’t want to jinx anything. We’ll see what happens in January, but we do have a lot of ideas if we get to do another one. [There will be] a lot more Subsiders if we get to do another one.

To read the full interview visit shocktillyoudrop.com.