The effect would be too substantial to be viable. Surprisingly poisoning people in mass is not as simple as it might seem. A dose powerful enough to ensure 100% death would have a noticeable and immediate effect on 50% of the population. Tipping their hand. Supposedly the gene manipulation will have a more gradual effect, but clearly, they need you to keep taking it, so it’s effects are diminishing, and not as powerful as The Science ™ would have you believe. My thought is that it’s two-pronged. Gene therapy for the obedient masses that perpetually weakens their immunity, with a hot shot mixed in to speed it up just below the threshold of public awareness.