First when I heard about 20M doses in December, I thought that was a lofty goal. Today I decided to research how many flu shots occur each year and in what time frame. From August through December 192.3 million doses of flu vaccine were distributed, with an average of 19.1M per week in the month of September.
https://www.cdc.gov/flu/prevent/vaccine-supply-distribution.htm
So, 20 million over 3 weeks seems completely doable.
Now, I know there are some storing and handling complications with the Covid vaccine, but it seems that there is already in infrastructure set up to distribute a large number of vaccines during ~4 months a year. Why aren't we tapping those same resources?
Also, for as much as we hear about super cold storage for the Pfizer vaccine, it has to be thawed before it goes in someone's arm, and from what I've read it still has a shelf life of 3-5 days at refrigerated temps. So while the storage does complicate things, I don't think it's to the degree (pun intended) that some make it out to be.
I think two things have slowed it down.
The first is one of the things I've complained about here before, which is that we're spending too much time with the vaccine sitting in freezers while we make sure that we administer it in the precise order that we, as a state, have decided that it should be administered.
Second, I think that if a state gets say 100,000 doses, some states are only looking to vaccinate 50,000 people, holding back half to do the second dose. My personal preference would be to get the first doses into people and use future deliveries for the second shot.