I've just watched Coppelion. Was curious about the manga, and according to the Wikipedia page... it is still going on? Apparently it has nearly 200 chapters, so I guess that the anime doesn't cover a large portion of the story.
The Coppelion manga is broken down into "arcs" (部). As of this writing, the manga is part-way through the fourth arc, while the anime finished up at the end of the second arc. So the anime covers roughly half-ish of the manga released so far.
How faithful is the anime, so far?
The anime was generally very faithful to the manga. There are minor differences in the order in which things are presented, and there's a flashback featuring Naruse and Haruto in Mission.40 which I don't think appears in the anime, and some of the stuff towards the end of the second arc are kind of rushed, but all things considered, there aren't really many differences.
If you want to verify for yourself that the anime is highly faithful, see below for a table indicating which chapters corresponded to which episodes. Note that Coppelion has a strange chapter naming scheme, in which:
- The first arc has 21 chapters, numbered "#1", ..., "#21", and not given names.
- The second arc has 60 chapters, numbered "Mission.1", ..., "Mission.60", and given names.
- The third arc has 99 chapters, numbered "Phase.1", ..., "Phase.99", and given names.
- The fourth arc (in progress) has 12 chapters (as of volume 20), numbered "Act.1", ..., "Act.12", and given names.
Episode Chapters
1 # 1-# 3
2 # 4-#11
3 #11-#16
4 #17-#21
5 Mission. 1-Mission. 5
6 Mission. 6-Mission.11
7 Mission.12-Mission.15
8 Mission.16-Mission.21
9 Mission.22-Mission.25
10 Mission.26-Mission.34
11 Mission.35-Mission.47
12 Mission.47-Mission.53
13 Mission.54-Mission.61