In a Nutshell: The CW is trying out the "limited series event" terminology with Julie Plec's Containment, a show about an epidemic striking the city of Atlanta and the quarantine that follows. It will get six weeks after the network's biggest lead-in The Flash, then air the second half of its season in the summer.
Best Case: It is pretty egregious how long the CW stuck with the incompatible iZombie in the post-Flash slot. It definitely has its own audience, it's also true that practically anything would do better in this timeslot simply by benefiting from The Flash's viewership. Even merely decent retention puts it well above iZombie's season high tonight, and it holds to most of it at least for the rest of the regular season. 0.80. It drops a bit after the regular season ends, but is still a pretty clear success and a renewal.
Worst Case: Maybe lead-ins just don't matter with young-skewing shows like the CW dramas, and it actually is possible to get completely rejected by The Flash's audience and do far worse than iZombie. It starts at the low end of iZombie ratings and ends its regular season run with a bunch of 0.3's... even with the Flash lead-in! 0.35 for the regular season, and never any better than a 0.2 during the summer.
Likeliest: On the one hand, this is very late in the season to be premiering something that might have a long-term future. On the other, it is getting The Flash lead-in, so the network probably isn't as down on it as they were with something like The Messengers last year. And any comparison with iZombie is weird since that show seems utterly unchanged by any lead-in. This concept isn't appealing to me, but I'm guessing it'd be tough for anything to do much worse than iZombie did with a Flash lead-in. I think it premieres at 0.6-0.7 tonight, has several weeks with iZombie's usual 0.5, and throws in a couple 0.4's toward the end of the season for a 0.48 average for just the regular season eps. But the verdict will be based on what happens after the regular season and The Flash leave the picture. And at that point, I think it will become clear it has far less audience than iZombie; it instantly falls to 0.3 or even 0.2, and the network will not continue with it. They've renewed enough shows already!