Senior citizens in nursing homes are relatively few in number yet very high in risk of complications and death from COVID, so they should be first. You can't just go at the great body of the population when you don't have but a small percentage of doses for them and you can't give them out very fast yet. Take care of the elderly and the death toll will be slashed in half or more. Then you get the health care personnel, who are in the multi-millions and who are at high risk of contagion and transmission. Then go after people 50-64 and those with comorbidities. From that point on, you'll have a tiny fraction of the death toll and you'll have much more supply and better dissemination, so you can tackle the 150-200 million remaining to get to herd immunity.