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Not trying to be snarky, but when the rules say you can. Vaccine doesn’t stop transmission, just significantly reduces chances of getting severe disease requiring hospitalisation, and as others have said travelling to/from etc increases risk of transmission. Until enough of the population has had the vaccination to get proper herd immunity and we don’t run the risk of shafting the NHS again, we need to adhere by prevailing precautions. In terms of personal risk to your grandma, she should get decent partial protection (50-60%) AFAIK after about 7-10 days after first dose, and then maximum (but not complete) protection from severe disease roughly the same time again after second dose.
Sorry if this is covered elsewhere - my grandma just had her first dose of the astra zeneca vaccine - when can I go round for a cup of tea?