I can't post the link for some reason but google 'JCVI third vaccination' and there's a gov.uk page.
I've cut and paste the relevant section below:
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Advice
At the current time, JCVI advises that a third primary dose be offered to individuals aged 12 years and over with severe immunosuppression in proximity of their first or second COVID-19 vaccine doses in the primary schedule. Severe immunosuppression at the time of vaccination is defined using the guidance and timings stated below.
- Individuals with primary or acquired immunodeficiency states at the time of vaccination due to conditions including:
acute and chronic leukaemias, and clinically aggressive lymphomas (including Hodgkin’s lymphoma) who were under treatment or within 12 months of achieving cure
individuals under follow up for chronic lymphoproliferative disorders including haematological malignancies such as indolent lymphoma, chronic lymphoid leukaemia, myeloma, Waldenstrom’s macroglobulinemia and other plasma cell dyscrasias (note: this list is not exhaustive)
immunosuppression due to HIV/AIDS with a current CD4 count of 3.0mg/kg/day, 6-mercaptopurine >1.5mg/kg/day, mycophenolate >1g/day in the previous 3 months
certain combination therapies at individual doses lower than above, including those on ≥7.5mg prednisolone per day in combination with other immunosuppressants (other than hydroxychloroquine or sulfasalazine) and those receiving methotrexate (any dose) with leflunomide in the previous 3 months
- Individuals who had received high-dose steroids (equivalent to >40mg prednisolone per day for more than a week) for any reason in the month before vaccination.
Individuals who had received brief immunosuppression (≤40mg prednisolone per day) for an acute episode (for example, asthma / COPD / COVID-19) and individuals on replacement corticosteroids for adrenal insufficiency are not considered severely immunosuppressed sufficient to have prevented response to the primary vaccination.