New Badge for nations actively participated to Zombie Apocalypse, possibly requiring some objectively measurable result or combination of results, For example minimum amount of Actions taken against the Zombies or for 'Zombification' with corresponding survival rate / amount of survivors or minimum omount of "infections caused" etc.
If Z-Days are going be annual happening then there could used similar system with these badges like is currently used with Easter eggs.
That way each nations' actions, outcomes and results would leave permanent mark, - "stain" or award -, commemorating Z-Days there after.