Good question, I wish I had the definitive answer. I'll see if I can find anything and ping you if I do.
http://www.sciencedaily.com/releases/2003/11/031112073642.htm
This article indicates that at least some immunity is retained for over 50 years.
From what I remember when smallpox was the big focus in 2002, there is a residual immunity that persists but decreases with time. I’m not sure any definitive studies have been done since smallpox was supposedly eradicated in the 1970s.
Also depends if it is an engineered variety or not. For instance, natural smallpox required close contact for transmission. The engineered variety (the kind that infected people downwind from Resurrection Island on the Aral Sea) supposedly infected people miles away. North Korea maintained a huge stockpile of these Soviet biological agents (tons). Iran may also have them as well. After 9/11, people were concerned about stories Iraq had tried to obtain the virus as well.
I think you would need to know the structure of the engineered virus used as a bioweapon to know how much overlapping immunity would apply. No one in the U.S. (non-military) born after 1972-3, when vaccinations ceased, has any immunity whatsoever.
Ft. Campbell, KY is a massive military base. Clarksville, TN is just down the road.