I don't think the shots that have been administered are that deadly on thier own. I think the intention was always to cause mild damage that accumulates with each dose. That way the stragglers get at least a few shots in before the early adopters die in such quantity that it can't be hidden any more.

Potentially there is also an effect on the eggs that will make the offspring sterile. That will only become apparent in 20 years when the mudblood babies can't concieve. But that is not the same as a die off.

There is no magical way to make the shot very deadly, but not kill quickly. And (((they))) can't kill quickly without being exposed. They are experts at cooking the frogs slowly.