I got the impression it was just another dose of the same vaccine(s) already available rather than a new thing. I heard something about tests on vaccine combin

I got the impression it was just another dose of the same vaccine(s) already available rather than a new thing. I heard something about tests on vaccine combinations (i.e. different types of vaccine for your 1st & 2nd dose) a while back though but I can't remember the details, does anyone know what happened with that?