Government guidance is that Covid vaccination boosters should be given using Pfizer vaccine even if the first vaccines given were Astra Zeneca.
This is considered to be completely safe and effective.
The only exception is where someone had an allergy to the Pfizer vaccine.
In this situation you should discuss this with your GP.
For more information, please visit: www.gov.uk/jcvi-statement-regarding-a-covid-19-booster-vaccine-programme-for-winter-2021-to-2022
Check here to confirm you are eligible for a booster: