BCG seems to prime the immune system in non-specific ways, although IIRC the exact mechanism is still not well understood.

There is a trial in New Zealand, I think, to test whether it gives protection or not. Not sure if they'll be able to finish, given the virus was aggressively stomped there.

It’s also used in low-level bladder cancers as a wash of the bladder. I think it’s a similar idea in that it helps provoke an immunology response that’s been found to be helpful.