Posting this again as there still seems to be confusion.
In England at least, vulnerable people are getting a Third primary dose now as well as a Booster. The third primary dose is not the booster, even if your GP surgery may call it that. I had my my 3rd primary dose a couple of weeks ago, 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 had a 3rd dose, though it was 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., there hasn't been much comment on this in the media, but surely if GPs have read what they should, they would know. I will be eligible for a Booster in 6 months as well, but the 3rd 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. It has been decided to give a different type for 3rd primary and Boosters - Pfizer when my first 2 were Astra Zenica.
Here's a link to the relevant Government advice. Our situations are covered mostly in Advice para 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.