You're considered clinically extremely vulnerable if:
your doctor or GP has classed you as clinically extremely vulnerable because they think you're at high risk of getting seriously ill
you've been identified as possibly being at high risk through the COVID-19 Population Risk Assessment
you've had an organ transplant
you're having chemotherapy or antibody treatment for cancer, including immunotherapy
you're having an intense course of radiotherapy (radical radiotherapy) for lung cancer
you're having targeted cancer treatments that can affect the immune system (such as protein kinase inhibitors or PARP inhibitors)
you have blood or bone marrow cancer (such as leukaemia, lymphoma or myeloma)
you've had a bone marrow or stem cell transplant in the past 6 months, or are still taking immunosuppressant medicine
you've been told by a doctor you have a severe lung condition (such as cystic fibrosis, severe asthma or severe COPD)
you have a condition that means you have a very high risk of getting infections (such as SCID or sickle cell)
you're taking medicine that makes you much more likely to get infections (such as high doses of steroids or immunosuppressant medicine)
you have a serious heart condition and are pregnant
you have a problem with your spleen or your spleen has been removed (splenectomy)
you're an adult with Down's syndrome
you're an adult who is having dialysis or has severe (stage 5) long-term kidney disease
https://www.nhs.uk/conditions/coronavirus-covid-19/people-at-higher-risk/who-is-at-high-risk-from-coronavirus-clinically-extremely-vulnerable/