The simplest argument for First Doses First (FDF) is that 2*0.8>.95, i.e. two vaccinated people confers more immunity than one double vaccinated person. But there is more to it than that. Perhaps more important is that with FDF we will lower R more quickly and reach herd immunity sooner. Here’s an extreme but telling example.
Suppose you have a pop of 300 million, need 2/3 to get to herd immunity and you have 100m doses and can vaccinate 100m a month. Then with FDF you vaccinate 100m in first month and a new 100m in the second month and then you are “done.” i.e. you can then do 2nd doses more or less at leisure since you are at herd immunity (yes, I know about overshooting, this is a simple example). If instead you do second doses you vaccinate 100m in first month and the same 100m in the second month which leaves 100 million at risk for another month. Under second doses you don’t reach herd immunity until the third month. Thus, under FDF you save a 100m infection-month which is a big deal.
Now when you put this into a more sophisticated SEIR model you won’t get as strong a result but the result will be in the same direction. Note also that getting to herd immunity sooner is probably the best thing we can do to prevent further mutations.
See also Youyang Go’s thread where he discusses his modeling of similar ideas. He notes:
Reaching herd immunity two or three months sooner will have profound benefits throughout society, ranging from fewer cases & deaths to faster economic recovery.
Addendum: Please read Tyler’s post, FDF?-Show Your Work! before you comment.