Let's say your the DS, you want to kill 90% of the population with a deadly bioweapon but if you release something that incredibly virulent you risk killing all life on the whole planet.

So what do you do? You, split up the recipe. You create a less severe "pandemic" get all the humans to take the first part of the recipe(the shot).

You then later release a reactive agent that kills anyone who got the shot in one fell swoop or blame it on another strand of virus (reactive agent) that you created to do the job.

Game, set, match. The lemmings do all the work for you as you sit back and watch the shitshow; sipping your sangirita on the veranda.

Depending on how long they have had to develop Covid19. There could be multiple objectives and contingencies caked into the virus and or "vaccine".

Just a little food for thought before you decide to get the jab.