there isn't a master list of all people vulnerable to work from.
Exactly. There are all sorts of branches of the NHS including GP's practices, trying to create lists of people who are particularly at risk if they got Corona-virus, warn them and offer them help to safely self isolate if they need it, as right now it's actually the only answer we have.
Inevitably there are some people who got added who actually don't need to be, and others who do need to shield who aren't yet on it.
As you can see on the Gov. form that's linked to above the list of main conditions you can self register under seem quite random to most.
There are also other non self registering systems that allow for other conditions.
The aim is to try and give the 'known to be truly vulnerable' as good a chance as possible if they want it, and if they need it, then help to safely shield themselves.
There really is a very big difference for some, between getting this virus and other illnesses.
It's obvious I think why people with severe asthma are on it.
Perhaps less obvious why certain types of lung condition but not others etc.
If you've had a letter or get a call and you think it's an overreaction, please ask your Dr to review why. They may tell you it's fine.