I thought I'd post a new comment to help clear up any confusion.
A third primary dose is not a booster, even if your GP surgery may call it that. I've just come back from having my 3rd primary dose, though when the surgery booked me they called it a booster. Those of use who have PMR and who have been on Prednisolone for several years and who, like me are still on 10mg or more are regarded as Immunosuppressed. This is why I have been done today, though it's only 5 months since my 2nd jab. The Dr. who did me said this is a new development in the Covid strategy. It's not surprising that there is confusion about terminology. I haven't seen any comment on this in the media. I will be eligible for a Booster in 6 months but today's jab should be seen as a continuation of the original protection given by jabs one and 2, hence the correct title being 3rd Primary dose. I've had no explanation as to why this was Pfizer when my first 2 were Astra Zenica.
Here's a link to the relevant Government advice. Our situations are covered mostly in paragraph 3.
gov.uk/government/publicati...
It was also mentioned that my being on sulfasalazine was a factor, though this govt. info says otherwise.
I hope this helps.