Thanks for the nudge, Alex.
Dr Poland addressed your question (sort of) in this podcast.

Excerpt
Dr. Halena Gazelka 10:26
Excellent. All right, our next listener asks. They state that they are one of the immune compromised individuals in the moderately to severely immunocompromised category who got a third dose of Pfizer back in August when it was authorized. It wasn't called a booster, it was called a third dose. So, does that mean that they get to then have a booster as a booster dose I guess? So, a fourth dose it would be.

Dr. Gregory Poland 11:05
Yes, and this is something that is, you know, you talk about public health messaging, it's something that is very confusing. So, let me go over it carefully. This is only pertaining to people who are moderately to severely immunocompromised. Okay. If you got an mRNA vaccine, you would have gotten two doses, the recommendation was that 28 or more days later, you get a late dose or a third dose, as you mentioned, Halena. Excuse me. And then six months later, you get your booster, i.e., your fourth dose. If you started with the J&J vaccine, then you get your first dose, and then two months later you get your second dose, stop that's the end. So, with J&J, it's a total of two doses with the mRNA vaccines to a total of four doses. A little confusing, I recognize.

He also talks about boosters and the J&J vaccine in this next podcast