CLL Support, the parent charity of this site, has posted very useful information about the difference between a booster and a third dose. There's also a lot of general information about covid vaccine safety in general and, specifically, the risk of having a clot.
The third dose for the immunocompromised is an addition to the primary course (first two doses of the vaccine) to try and raise the level of immunity among the immunocompromised to match the immunity of the rest of the population.
Booster vaccines are likely to then be given on top of this in the New Year, approximately 6 months after the immunocompromised have received their third dose.
cllsupport.org.uk/covid-vac...
Jackie