It'll kill a lot more people due to the devastation of Europe being far greater than in 1918. Also with a lot of the Troops returning home just like in 1918, it'll spread like wildfire. Except this time it will spread a lot further.
Yes, it's honestly a bit surprising that we didn't have a plague post ww2. We certainly provided nature's bioweapon R&D department with tons of test subjects under near ideal conditions.