After ’99, even the Vatican respects this particular rule. (It helps that the current head of Section XIII is more reasonable than Maxwell.) When they discover that a vampire target is working with a human, they halt all operations and call in the Protestants.
Of course, most of the Iscariots are still not thrilled about this state of affairs. Which is why the shorthand for this is “code I.F.H.”: “[oh, great, now we have to stop everything while we call] Integra F@#king Hellsing.”
Seras: I know it’s not funny, but…it’s just so much less serious than the worst-case scenario. That’s why we got called in, you know. Human-vampire partnerships automatically fall under our jurisdiction. Especially considering when Stephen was turned.
Jon: If you think either of us is anything like the traitors of ’99–!
Stephen: Jon, please stop antagonizing her! She knows, okay? She bit me, and it hurt, and — she knows our history. All she’s trying to do now is get a read on you.
Seras: So far, I’ve worked out that you have balls of steel.
Jon: Eh. I’ve been sure I was going to die at least three times tonight. By this point I’m just numb to the idea.