Some people have water filters, others drink bottled, some drink more than others, additionally you would have to do all regions of the world simultaneously, also it's not easy to determine when to keep adding poisons and when to stop, etc.. If you really think about all other methods, I mean really really take the time to think of how to do it on a mass scale, then a vaccine with delayed effect is the most practical. (Also starvation and chaos is a pretty good option as well)

even so this implies direct involvement is dose selection by healthcare. This opens them up to liability and increases the surface of secrecy. We would have had leaks by now

Different batches of "vaccines" do not need any involvement of medical care personnel, they just receive the batch and inject it, they do not know what they got. In many places of the world you take an appointment and they tell you where to take your poison (different centers can have different batches), it also depends on the location of the distribution. Of course when they wanted to get more numbers they discarded the appointment system, and could only control by location. It's not perfect but better than nothing.

People are highly specialized in their work, they do not really need to know much, and they are easily suspect to bribe and to obeying orders, never count on any "leaks".

Some people have water filters, others drink bottled, some drink more than others, additionally you would have to do all regions of the world simultaneously, also it's not easy to determine when to keep adding poisons and when to stop, etc.. If you really think about all other methods, I mean really really take the time to think of how to do it on a mass scale, then a vaccine with delayed effect is the most practical. (Also starvation and chaos is a pretty good option as well)

even so this implies direct involvement is dose selection by healthcare. This opens them up to liability and increases the surface of secrecy. We would have had leaks by now

Different batches of "vaccines" does not need any involvement of medical care personnel, they just receive the batch and inject it, they do not know what they got. In many places of the world you take an appointment and they tell you where to take your poison (different centers can have different batches), it also depends on the location of the distribution. Of course when they wanted to get more numbers they discarded the appointment system, and could only control by location. It's not perfect but better than nothing.

People are highly specialized in their work, they do not really need to know much, and they are easily suspect to bribe and to obeying orders, never count on any "leaks".