I think "close borders till vax" is probably the least-bad COVID policy overall, if it can be done. I do think that it would have been tricky to do in the UK though.
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We get a lot of lorry freight through the Channel Tunnel. We would have had to have worked out a way to quarantine or keep out huge numbers of drivers, many of whom are obviously British. It's a bit different with boat freight---container ships use few people these days, and the small staff numbers can quarantine on their boats which are fully equipped for this.
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We are not an island and we share a soft border with RoI. Either we would have had to have turned this into a hard border, or we would need to draw a line around the island of Ireland, cutting off Northern Island (which would have enraged the Unionists). Both politically very difficult, though perhaps not impossible.
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Back in spring 2020, overwhelming medical opinion (WHO et al) was that border controls don't work and are counter productive, and very few people would have supported it. Business interests would have fought tooth and nail. Most of the left wing press would have exploded with rage---"it's racist," "it's xenophobic," "it's unfair to immigrants and diverse families with relatives overseas," "it's insensitive towards the Irish," "Boris Johnson is only doing this to try to get the ground prepared for a hard Brexit with a hard border in Ireland" etc. etc.
Remember, right now we have a political alignment where people on the left are (for once in their lives, in most cases!) suddenly in favor of hard border controls while more conservative people are more likely to be against this. But this was not the case in spring 2020. Back then, it was Nigel Farage (!! Yes, really) howling that we should close the borders to keep the dangerous Chinese disease out. The left leaning media was full of cheery content that was airily reassuring us that we should keep open borders, that we shouldn't "blame germs on foreigners" and there was initially tut-tutting about Australia closing its borders. Everyone seems to have conveniently memory-holed this, but the reality is that closing borders would have really politically difficult at the time, and huge numbers of people would have basically refused to comply.
"Almost no one ever talks about the great covid realignment, completely forgotten to time" See this for some eye-popping tweets, including the one from dear old Nigel Farage.
twitter.com/edwest/status/1318460476901253120