I had a smallpox shot as a kid and it never scarred either.
When I went to college, it was required that I have a "visible" smallpox vaccine scar. A physical was also required back then. Went to my dad's internist and he read the requirements on the form, so said he'd give a vaccine to make sure that I had a scar.
Yeah....he gave me a vaccine alright... I was sick as a dog for a couple of days, couldn't lift my arm for about a week! And....I still don't have a scar!! 
DEBK, we are still trying to find out how the body works.
I read a few years ago a medical report from Europe on why some people in the high risk category's never get AIDS.
At that time scientists felt that people with immunity to the disease had ancestors that survived the Black Plague. Milk maids that contacted the cow pox seldom got small pox.
Today we know that people get shingles if they have had chicken pox, Nurses and doctors that never had chicken pox can tend to those that have the ailment without getting shingles themselves.
The jury is still out about vaccinations causing autism in one child but the next sibling has no problem. Genitics?
Unless one is an identical twin--even then their finger prints are different, the DNA is different in everyone.
We just do not know what kind of diseases our ancesters survived to cause problems in future generations.