Complex sentence: Since human plague is rare in most parts of the world, routine vaccination is not needed other than for those at particularly high risk of exposure, nor for people living in areas with enzootic plague, such as the western United States.
Split into simpler sentences: Since human plague is rare in most parts of the world, there is no need to vaccinate people other than those at particularly high risk of exposure. Routine vaccination is not necessary for people living in areas with enzootic plague such as the western United States.