Cat flu in past, do you let them go outside?

Just a question, I have a cat that had cat flu before we brought him home, and is indoors only.
Sometimes I feel sorry for him being indoors only. He is up to date with vaccinations etc.
Does anyone have a cat that had the flu in the past would you let them outside? I understand that they can be carriers. I don't want to let him outside but I do feel for him, as we have cats around here.
Do you know anyone that lets there's outside? I don't think it's a good idea to be honest as they can pass it around. Especially if they roam around the neighbourhood.

But I also wonder how does it get passed around ? Some people must let them outside when they had cat flu before.

What about training him to walk on a harness and taking him for walks -- that would satisfy his need for sunshine without endangering anyone else's cats.

Ok might be a good idea, how would I go about that?
We do have some neighbour cats that do come in our backyard at night though.