From reader Willyvictor:

Once upon a time, a bunch of scientists and government wanted to develop a biological treatment that allowed you to use cell technology within the body itself to deliver medication or other instructions. The problem is how do you test such technology? You see, to make this work it is necessary to inject substances like graphene-oxide into the body. 

Such substances are toxic so you are not quite sure how to mask these substances. This requires changes in formulation which means multiple injections. 

All of which creates a problem because your test subjects will experience adverse reactions. The reactions will be debilitating to some extent and fatal to some subjects. How on earth can you ever find enough test subjects to accomplish your goal?

A global pandemic is your excuse to inject millions and provide cover for the injuries and deaths that result from your experiments. Subsequent injections are called booster shots as you make changes on the fly and adjust in real time. The results are tracked.

Eventually, you perfect your technique. The dead and injured are of no consequence because the system has to start with a smaller total number. Also, interaction with the untreated tends to modify the effect of the treatment. Therefore, it is better to cull the heard, so to speak and end up with a more manageable number of fully-treated subjects.

Subjects: those who do what you tell them to do.