Why do some who get the vaccine have no adverse events?
Gnarls. 
How is it some people swell up and nearly, or die, at the mention of a peanut or shrimp? Most of us can scarf down peanut encrusted shrimp with no effects other than a full stomach.
My platoon hated me for a long time after night land nav excercises because, well,
I'M immune to Poison Ivy, Sumac, Oak, etc.. Some of them still think I did that purposely.
The immune system can be quite a fickle thing.
Applies to "batch related" issues as well. Animal physiology is still a mystery for the most part. This last round of medical science has definately show that "one size" does NOT fit all.
