[A brief, dark expression crosses her face, but it's gone quickly.]

We'll still need to do something else. I realize I didn't explain before, but...the last time a vampire subsisted off an elf's blood, the sunlight in their veins turned them into a shambling monster, a daywalker with no mind and no conscience. Tempering that with other sources should prevent the madness, and I don't know what me giving it willingly changes, but.

[She pauses, and rolls over to sitting up, which conveniently means he can't see her face.] As I said. It should mean you never have to take enough to kill, again.