Adeno virus based vector vaccines are a pretty old idea.

I was under the impression that adenovirus vaccines have only been been approved in the past year or two - a few for Ebola and the new ones for Covid. Are there others that have been around before that?

It doesn't seem like it really matters if adenovirus vector vaccines is an "old idea" - after all, the idea of using MRNA has also been around for a long time. But if we've only started actually using them recently, how different is our knowledge of long-term effects?

They are tested and researched since at least 2004. Not for Covid-19 of course.

And mRNA vaccines were first tested in the 90s and started getting more substantial research in 2008: the timelines seem similar?

Your impression is correct.

The fact that it is an "old idea" is indeed unimportant, but what is very, very important is that large scale study over long periods has already been performed on that vaccination platform so we really do know a lot about it.

That means that the similarity to mRNA is a good thing in terms of any worries about unexpected consequences. Of course it also means that we know that the risk of any unexpected consequences for adenovirus vaccines is also very low.