I also used to ascribe to the belief that rugby had fewer concussions (as a collegiate/semi-pro rugby player) but I wasn't aware of these data, thanks for sharing.

In retrospect, it makes sense to me. I think the nature of rugby collisions and how you play the game just lends itself to more concussions than American football. A lot of AF is throwing passes which get broken up by less violent hits versus in rugby the only way is to tackle the guy carrying the ball.