The theory of measles elimination: implications for the design of elimination strategies.

The theory of disease transmission provides a consistent framework within which to design, evaluate, and monitor measles elimination programs. Elimination of measles requires maintaining the effective reproduction number R at <1, by achieving and maintaining low levels of susceptibility. The essential features of different vaccination strategies (e.g., routine versus campaigns, number of doses) can be compared within this framework. Designing an elimination program for a particular population involves setting target levels of susceptibility, establishing the current susceptibility profile, selecting an approach to reduce susceptibility below the target, and selecting an approach to maintain susceptibility below the target. A key indicator of the sustainability of an elimination program is the residual level of susceptibility of a cohort after it has completed its scheduled vaccination opportunities. This can be estimated from vaccination coverage data. The high transmissibility of measles poses a significant challenge to any attempt to eliminate it.