I assumed we wouldn't be able to supply 100% of the globe at the same instant, maybe we'd distribute ~20% in a first wave (to the most vulnerable people). That

I assumed we wouldn't be able to supply 100% of the globe at the same instant, maybe we'd distribute ~20% in a first wave (to the most vulnerable people). That means that those who are rich[er] but don't meet the criteria are going to be in a position to want to buy. If the country/county/area/suburb next door is poor, and a locality has lots of cases then I'm guessing anyone who can afford it has an incentive to acquire a dose of vaccine early, or a booster dose, by buying out someone in that neighbouring locality. You need strong local government without corruption to distribute vaccines fairly.