Are shingles contagious to other people who have had chicken box previously in life but have a low immune system now?

How low?: You should have natural immunity if you had chicken pox, but depending on what is making your immne system "low", you could have some susceptibility. If you are going through current chemo, or high dose immune suppressive drugs, had total body radiation for something like a bone marrow transplant, could get the infection. If mild suppression of immune system, trasmission unlikely.

They can be: Coming in contact with a Shingles rash/blister can infect someone who doesn’t have immunity to chickenpox. That person could develop chicken pox (not shingles).