What scientific data have you analysed? Or have you read it on the internet?
There is not a snowballs chance in hell that an unsafe vaccine will be introduced. Some may be more effective in different age groups, dunno me. But the NHS will deliver the best vaccine for your grandparents, as long as they have enough of course. But thats a different issue.
My simple question at the beginning was if you walk in a room and get a COVID vaccination do you or can you find out which of them you're receiving. If people don't know the answer, that's fine. The likelihood is due to it being soon it will be the Pfizer vaccine but I was just curious if you can even find out which of them you get.