Major differences exist in the immunization programs of the United Kingdom and the United States. If one believes that most health policy decisions in Western industrialized democracies are political, then many of the differences may seem to reflect the variance in the nature of political systems. However, each program has unique components that appear paradoxical, and what works in one society will not necessarily work in another. Those who seek to substitute portions of one vaccine system with those of another must appreciate the context within which each functions.