As on Monday 11 Oct 2021, in Australia, for the immune compromised, well it list very detailed, only people on Chemo (or recently), Organ Transplant in the last 4 years, Renal Dialysis and some very rare auto immune conditions, to get 3 initial shots, those who got AZ, the 3rd must be Pfizer or Moderna, and the others must get Pfizer. (3 to be the same, Moderna has not been here long enough at this stage for these folks to be 2 shots up yet. Basically for simplicity I believe. ) In 9 months time, your AZ will be around half as effective as 3 weeks after your 2nd AZ shot, I would go it personally, but I suggest you seek highly qualified professional advice.