It's not a 'gamble,' it's more a calculated risk. This isn't something entered into lightly.
https://www.theguardian.com/commenti...d-dose-uk-jabs
And the commonplace suggestion that the UK is the only country pursuing this is way off beam as well. Here are the Dutch, contemplating TWENTY-FOUR weeks between jabs
https://www.dutchnews.nl/news/2021/0...cine-strategy/
That's based on Oxford/AstraZeneca but other countries are prepared to extend Pfizer to at least six weeks. Denmark for certain & France, Germany & Sweden at least all considering it. Hungary is even using the Russian Sputnik vaccine which hasn't been through anywhere near the same process as Oxford/AZ or Pfizer
In short every country is having to adapt its approach as none - beyond New Zealand and Australia - have the 'luxury' of time to vaccinate twice within short intervals. Given the problems now associated with supply this pressure is only going to increase. In short, with the possible exception of the USA, you're going to have look damned hard to find any country pursuing three-weeks intervals between vaccinations.
As I said earlier the vaccination roll-out so far is one of the rare UK successes and the reason for that success is that politicians of all parties have accepted without demur the recommendations of the scientists. In the case of Oxford/AZ we have a not for profit vaccine, efficiently distributed via a publicly owned health service on the basis of need.
If anyone had told me a month ago that nearly seven million people in this country would have had a first vaccination and close to half a million of them a second I'd have reckoned they'd had too much of the Xmas sherry.