the reason i say vaccine, its because vaccine made of weakened virus.
same reason, why hound is basically modified vector and can spread RDS too.
as for immune system : UNDO
since they defend the world by hunting both vector and hound
Vaccine isn't always a weakened virus. For example, mRNA is a synthetic RNA that instructs other cell to produce harmless protein, but it will trigger the immune response.
Hounds are NOT modified vectors, they are genetically modified "native lifeforms", as it was said in that chapter. RDS spreads not because of vectors specifically, but when system encounters something "unplanned" that it "can't undo".
UNDO is outside of that classification entirely. It's a human organization, they have no duty granted by designers of the system. Red blood cell can't become white blood cell just because it learned that body in danger and decided to help.
It's a matter of duty assigned to them. Vaccine is "look, i'm a scary virus, you should kill me and build defense so that others like me will not hurt you in the future". Hounds DEFINITELY weren't created for that, system can't modify itself in the first place.
Hounds were created for "hunt down everything that comes from outside to threaten the system". Bonafide immune system and can't be called anything but that. Problems comes only because it's addon thrown together in some garage as an emergency crutch, without having access to the system internals, but that's a different topic.
But okay, since they weren't a part of a system right from the start, it's indeed wrong to call them white blood cells. I guess you could call them antibiotics. And before "antibiotics can't kill viruses" argument comes up, we might as well call vectors bacteria instead, since the reason why antibiotic can't target viruses is theirs completely different non-cellular structure, while in our case vectors are definitely exactly the same kind of humans as non-vectors, just from a "different body" and therefore alien.
Also, UNDO aren't hunting down the hounds, heck they have a hound in their midst. They are hunting down an organization that decided to break the system. There is an important difference.
P.S. Actually, nevermind. Hounds were part of the system from the start, aka "native lifeforms", so no antibiotics. They are indeed an immune system, just the one brought into being by the forceful mutation of some original cells.