Clinically extremely vulnerable

Me, apparently, not young Dennis.

Dennis was out for a walk in his new outfit - such a well behaved dog!

I received an email from the NHS today which said I was CEV, with a suggestion I might get the covid jab soonish. I find it a little odd that when I was visiting Guys each week-day for five weeks, to have chemotherapy and radiotherapy, I was told I was not in the CEV category. My weekly blood test were good. Since 23/12 I have been recovering from the side effects of the treatment and am feeling much better. So, how is it that I am now CEV? 

Any ideas?