> A viral bioweapon is like trying to enact MAD by being the only ones with nukes and threatening to nuke everyone including yourself unless others do as you wish.
Someone with a viral bioweapon will develop it together with a vaccine. They will let only their people and allies have the vaccine, ensuring it is distributed secretly ahead of time, and wipe out everyone else.
Or possibly use it for blackmail: surrender everything to us, and those of you still left alive will be spared.
I'm worried that bioweapons at present levels of technology/understanding of science are just too risky for sane people to deploy. Even if a vaccine was paired with a pathogen, the risk of mutation making the vaccine irrelevant is too high.
Therefore, I'd expect one to be developed by some suicidal crazies with a grudge, who also won't have a particular need for a vaccine.
Someone with a viral bioweapon will develop it together with a vaccine. They will let only their people and allies have the vaccine, ensuring it is distributed secretly ahead of time, and wipe out everyone else.
Or possibly use it for blackmail: surrender everything to us, and those of you still left alive will be spared.