On December 30, 2020 Boris Johnson gave a speech in which he announced that the UK would follow a COVID-19 vaccination dosing schedule with a twelve week gap between first and second doses, instead of the 3 weeks that is standard in the US (and the rest of the world? [edit: Orual reports that Canada also used a dosing schedule that prioritized first doses]).
And there is one important development that’s helping us to accelerate our vaccination programme across the whole of the UK:
We’ve had new advice from the Joint Committee on Vaccination and Immunisation that the first dose can protect people against the worst effects of this virus because the benefits kick in after two or three weeks.
And so from now on we will give a first dose to as many vulnerable people as possible, with the second dose to follow twelve weeks later.
And what that means is we can vaccinate and protect many more people in the coming weeks.
As near as I can tell, this is a straightforwardly good policy in terms of public health, because it increases society's total heard resistance to the virus faster. But it also entails deviating somewhat from the specific conditions of the trials in which the vaccines were tested. If it turns out to be a bad idea somehow, you can't fall back on saying that this plan was standard practice, and you couldn't have done any better." So in some sense, it's a risk.
I would really like to know what causal factors were upstream of this decision. Why did this happen in the UK, but not in the US or elsewhere?
Are there specific individuals, in the limelight or behind the scenes that pushed for this outcome? Is the difference that, in the UK, someone was in the right place at the right time?
Is this a cultural thing? Does the UK government feel more comfortable choosing policies that make analytic sense, but which don't have the social stamp of approval of "The Science"?
And if it is cultural, what is the difference? Do UK politicians reason from physical models of the virus more than US politicians? Does the UK government expect that if they're attacked for a decision like this, that voters (or someone else?) will back them up, while that's not the expectation in the US? Does the UK government feel like it has credibility, and therefore can make strong, reasoned, recommendations, while the US feels like it doesn't have the slack to do that? 
Insofar as Britain was marginally more sane, on a societal level, than the rest of the world, it seems important to find out what made the difference.
Some additional relevant information:
Wikipedia says that there was some push-back against this decision by the British Medical Association, and Brittan's Chief Medical Officers published an open letter defending the decision.
That open letter is here.
It briefly expresses a simple argument for prioritizing first doses, of the sort that I and most Less Wrong readers would make. (There is a limited amount of vaccine; most of the immunizing effect comes from the first dose; it is more beneficial to public health for a marginal person go from 0% immunized to ~75% immunized than to have one go from ~75% to ~90%; therefore, given the tradeoffs, it makes sense to focus on first doses for the time being.)
We recognise that the request to re-schedule second appointments is operationally very difficult, especially at short notice, and will distress patients who were looking forward to being fully immunised. However, we are all conscious that for every 1000 people boosted with a second dose of COVID-19 vaccine in January (who will as a result gain marginally on protection from severe disease), 1000 new people can't have substantial initial protection which is in most cases likely to raise them from 0% protected to at least 70% protected. Whilst the NHS, through all of your work, has so far vaccinated over 1 million UK patients with a first dose,approximately 30 million UK patients and health and social care workers eligible for vaccination in Phase 1 remain totally unprotected and many are distressed or anxious about the wait for their turn. These unvaccinated people are far more likely to end up severely ill, hospitalised on in some cases dying without vaccine. Halving the number vaccinated over the next 2-3 months because of giving two vaccines in quick succession rather than with a delay of 12 weeks does not provide optimal public health impact.
This letter seems notable to me for being basically lucid and sensible, in contrast to much of what the CDC, for instance, has said this year.
“There’s risks on either side,” Anthony S. Fauci told The Washington Post, warning that shifting to a single-dose strategy for the Pfizer-BioNTech and Moderna vaccines could leave people less protected, enable variants to spread and possibly boost skepticism among Americans already hesitant to get the shots.
“We’re telling people [two shots] is what you should do … and then we say, ‘Oops, we changed our mind’?” Fauci said. “I think that would be a messaging challenge, to say the least.”
(Note that the quoted article was published in March, more than two months after the UK announced their dosing schedule. Switching in March does seem more confusing than using a 12 week schedule from the get go.)