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Unless I've missed a change, the advice for this one is that once you've been vaccinated you need to be kept under observation for 15 minutes. If you're vaccinating 1 person per minute (which is the rate they managed for flu at our local surgery - in six vaccination stations around the surgery ) that means that for each vaccination station you need a very large holding pen for 15 grumpy adults kept 6 foot apart from each other, and in most places also somewhere to hold and move a hundred or so cars.
My local surgery essentially did most of the flu vaccine process outdoors - the only bit that was actually under cover was the jab. Fortunately on the day I went it was dry.
My eldest daughter (NHS Nurse Practitioner), has been doing Covid Vaccinations for a few weeks now (ie, since Pfizer vaccine approval). They are doing them in former walk-in centres (now defunct), in the area where she is working. She is doing 1 day per week vaccinations and 4 days per week her "normal" job.
I spoke to her over the Christmas break, and, asked about the "observation period", as you say, 15 minutes "observation".
Until now, they have been doing those 80 and over, plus Health Workers etc.