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.
From my understanding of the Canada situation, it may have been motivated by less access to vaccines initially. The US did very well in terms of getting lots of vaccines soon (https://ourworldindata.org/covid-vaccinations) while Canada took about 4 months after the US to really get going. Canada may have been more desperate to prevent Covid (or have their numbers stop lagging the US), and thus been less risk-adverse.
This argument does not work for the UK, as they have been ahead of the US the whole time.
https://www.msn.com/en-ca/news/canada/vaccine-panel-says-canada-can-delay-second-dose-of-covid-19-vaccine-if-shortage/ar-BB1cIJaG This article cites the decision being partly justified by limited supplies and how bad things were.