I posted this on another thread a few days ago, the text is lifted from one of the Welsh Health Boards and explains in more detail:
❗ Timing of second doses (difference of Oxford AstraZeneca & Pfizer)
We are aware that lots of people have questions about the timing of second doses of the Covid-19 vaccine – and the difference between the timings of the Oxford AstraZeneca and Pfizer vaccines.
The Mass Vaccination Programme has started the rollout of Pfizer second doses and where supply is adequate we are looking to bring the interval forward from 11-12 weeks. We are aiming to administer the Pfizer second doses to all those who received their first vaccine between December 7th and February 21st by the end of March.
For those who received the Oxford AstraZeneca vaccine - the dose interval showing the highest efficacy is 12+ weeks and this timescale will be used, unless there are individual clinical reasons to use a shorter interval. The programme will commence the rollout of second dose of AstraZeneca appointments from March 29th.