The long rumoured Rainbow Six: Siege 'zombie co-op' spinoff has been confirmed at Ubisoft's E3 press conference, and is indeed called Rainbow Six: Quarantine.
Jade Adam Granger is the lead on this new game, and described the objective as being to create a new kind of co-op experience atop Rainbow Six: Siege's foundation. She ended on the line: "what Siege is to the fps shooter genre, we will be for PvE co-op."
Here's the trailer:
Interesting note: the game is three player co-op, in a genre where nearly everyone else goes for four. Little else to be gleaned from CG but, with a 2020 release due, it won't be too long before we see more.
As a bonus, Ubisoft has posted a video of Granger discussing the trailer and some of the game's mechanics: