In Call of Duty: World at War, it decreases bullet damage by 25% and does not have any effect against the Fireworks perk. The Flak Jacket perk was created to be the antithesis to Fireworks, making Juggernaut less powerful in Call of Duty: World at War. In almost all situations, Juggernaut will allow the user to survive one more shot. A Juggernaut symbol (a plus sign in a red and white circle) will appear just below the cross-hairs when a player hits an enemy player with the Juggernaut perk.
The perk symbol in the Wii version for Call of Duty: World at War is different from other console versions, it is a shield instead of a cross with a bullet over it. This is perhaps due to the symbol's resemblance to the Wii's D-pad, which caused it to be changed to avoid confusing the players.
Even though in Zombies the soda is named Juggernog, the textures for the label on the bottle name it "Juggernaut Soda". This was most likely the name used when the Perk-a-Cola vending machines were first being tested and didn't need to be changed, as it is very hard to read the bottle.