As far as I am aware, the UK was quickest off the mark to give it authorisation via the MHRA, hence it can start being given here. Trump turned down the offer of more vaccines, for financial reasons, I wouldn’t be surprised to hear.
Some of my friends have AC who have been working on the vaccine, having been ‘repurposed’ from their normal scientific work and another friend has been on the trials a d had two jabs with no ill effect. I also have a very well-qualified pharmaceutical chemist friend, who has worked across many continents, who feels that this has been well-developed.