Are booster jabs and primary jabs the same vaccine?

[CrumpledCrumpet]

Question for anyone knowledgeable.

I have a relative who has been offered a third primary jab because they are immune compromised. They’re getting a bit confused because they’ve already had a booster jab.

As far as I understand it, people are getting identical vaccines irrespective of that’s called their “booster” or a “primary”. So it doesn’t make any actual difference you get a booster before a primary or a primary before the booster - it’s the same thing.

Is this right?

The moderna booster is a half dose I think but the others are full dose.
What she had as a booster will now be her 3rd dose and then the next dose is her booster

Moderna is a half dose as a booster, a full dose as a third primary. Pfizer is the same dose for both.

Severely immunocompromised people don’t react as well to the vaccines as healthy people, so the amount of protection healthy people get from two jabs the immunocompromised need three jabs to get the same protection.

Then everyone gets offered a booster after 3 months.

The thing making it confusing is that the advice for the severely immunocompromised to get three primary doses only came out in September, when a lot of the severely immunocompromised were becoming eligible for the (then) six month booster. Anyone getting vaccinated for the first time now who is severely immunocompromised would get their first three doses about 8 weeks apart.