A hypothesis I had was that the US was sticking to an exact formula due to higher vaccine hesitancy, in order to “play it safe” and give less for anti-vaxers to criticize. After looking at a small handful of countries, I think this is not a significant cause of the difference in responses.
If this were true I would expect countries that have higher vaccine hesitancy to be less likely to do first doses first.
Checking [this data](https://www.thelancet.com/cms/10.1016/S0140-6736(20)31558-0/attachment/720358f5-8df0-405b-b06f-7734cf542a58/mmc1.pdf) which was near the top of search results, and using eyeballed values of strongly agree to “I think vaccines are safe” as the measure:
Canada: 75%, Yes FDF (March 3)
US: 66%, No FDF
Mexico: 60%, Yes FDF (Jan 22)
UK: 50%, Yes FDF (Jan 4)
Germany: 50%, Yes FDF (March 5)
Obviously a really small sample and I am being loose with the data, but it does not support this hypothesis, with no obvious correspondence between vaccine-confidence and when FDF started. I chose the countries in question off the top of my head.
Dates and sources were found by searching online, I have not carefully checked them.
https://www.statista.com/statistics/1195560/coronavirus-covid-19-vaccinations-number-germany/ This graph looks like there is about a 3-week lag in 2nd doses.
https://www.thetimes.co.uk/article/germany-follows-uk-by-delaying-second-dose-of-covid-vaccine-mk65kkh9w March 05, Germany starts FDF.
https://abcnews.go.com/Health/wireStory/mexico-russias-sputnik-shortages-limited-2nd-doses-77617433 Mexico does first doses first due to supply issues with the Sputnik vaccine; the first dose can be produced faster. The article does not mention the save more lives argument.
https://www.nasdaq.com/articles/mexico-may-delay-second-vaccine-doses-and-allow-private-orders-to-tame-raging-pandemic The tone of this piece seems to suggest FDF out of desperation.