There is still room for MRNA to sell 100 million doses to US, but they have to match the price (Trump is tough). If yes, then US is contracted to buy 300 million doses, enough for 150 million people, less than half of population, assuming all are approved.
US is hedging its bets on approval and can already get 600 million doses from PFE or GSK as long as one is approved. If 35 million refuse vaccination, one supplier already under contract can fill the demand.
Assuming efficacy and safety are neutral factors to preference, it then comes down to price and delivery schedule. As of now, Moderna looks on the short end of both.
BTW, US "bought" low dose price from GSK with development funding, which is risk money. Had Moderna done the same (as they already had the risk money), I'm confident they would have had the first contract. Now Moderna is on the short end of the selling stick in US, UK, EU and Japan.
A business needs to cover 3 bases to succeed: Product, selling and administration.
You can have the best product in the world but if you can't sell it or run the business efficiently, the business will fail.