Games : Preventing mobs from taking damage from explosions

I've been working on a concept for an arena-style players-vs-mobs mini game in vanilla minecraft. I'm looking for a way to keep mobs from taking damage from creeper explosions using commands and command blocks. Does anyone know of a simple way to do this? I've already tried placing non-player mobs on the same team and removing friendlyfire, but creeper explosions still kill other mobs.