You don't know that, it hasn't been around long enough. You don't have proof that it is harmful, but you don't have proof it isn't either.
I don't know it but I don't have to. I don't have to prove a negative. It's on you to show that it is harmful, otherwise it isn't.
I'm not sure anyone is able to conclude one way or the other. I'd like to make the choice about what my lungs consume and not have some person decide because they are ignorant.
Don't vape then. If someone in public is vaping near you, walk away.