It’s unfortunate that you have a high risk health condition. I’d be very surprised if they reject your request for a refund given their recent, very public, post to the contrary: https://www.casact.org/press/index.cfm?fa=viewArticle&articleID=4865 . In fact, if they reject it, please post their response, I’d be happy to join in with you in putting pressure on them to respect their own policy. Also presumably you’re in the US, so you have a very strong lawsuit if you choose not to take your exam and you’re fired.
Again, as all things in life, if doing something makes you uncomfortable, don’t do it. I think you have a lot of possible routes you can go down to solve this problem.