Up until now, people receiving the Moderna or Pfizer vaccines have been asked to wait for 15 minutes before leaving the vaccination centre. This was because the rate of serious allergic reactions (anaphylaxis) reported after these 2 vaccines is slightly higher (around 5 to 10 per million doses) than after other vaccines (normally 1 per million). However due to the Omicron variant, the booster programme for adults is being accelerated. As part of this, and given the very low rate of anaphylaxis, the 15 minute wait has been suspended, as we now have much more experience with giving these vaccines to millions of people.
If you have a history of allergies, particularly to other vaccines, or if you had an immediate reaction after your previous doses, you may be advised to stay for the 15 minutes. Please make sure you tell the vaccination centre. Note that a family history of allergies (or even anaphylaxis) is not a risk factor. Please also tell them if you have previously fainted following vaccination. Otherwise you will be able to leave the centre straight after your vaccine as long as you feel OK. You must not drive for 15 minutes after the vaccine – this is because of the risk of fainting.
More information can be found here