On the 24th December 2021 ATAGI recommends the following advice:
People aged 18 years and over with severe immunocompromise who have received 3 doses of
a COVID-19 vaccine are recommended to receive a booster (i.e. 4th dose) at 4 months, in line
with the timing for the general population. This is expected to improve protection against
symptomatic infection, serious illness or death from COVID-19 caused by the Omicron variant.
If you are unsure please speak with your healthcare team.
You can read the full statement here