Let’s just say for example sake you’re a gaggle of Eugenists and you want to reduce the birth rate of targeted, or all populations. You use a vaccine that will render 1-2 generations infertile but will fade or be adjusted later. You can’t take the risk of giving to everyone, the important, the wealthy, celebrity, political donors. So you issue those people, and a percentage of the population a placebo so they wont be affected.