All World Health Organization (WHO) Regions have goals to interrupt endemic measles circulation on or before 2020. This is a worthy, feasible (already achieved in the Region of the Americas) but formidable challenge, given the unique contagiousness of the measles virus, which demands homogeneous population immunity over 92% in most settings. High population immunity is necessary to ensure that the effective reproduction number is driven below 1, a pre-requisite for achieving and sustaining elimination.