In several cases, the dynamic consequences of these non-linear phenomena can have significant impacts on health outcomes and policy recommendations. Infection with rubella in childhood results in a relatively mild febrile illness, but women exposed to rubella in their first trimester of pregnancy are at risk for a variety of developmental defects collectively referred to as congenital rubella syndrome (CRS).  In the absence of vaccination, rubella infection is sufficiently common that only a small fraction of women remain susceptible by the age of first reproduction and rates of CRS are low.  Following the introduction of vaccination, the reduction of the force of infection and the concomitant increase in the mean age of infection mean that, while overall prevalence of rubella infection is expected to decline, the absolute number of women still susceptible at childbearing age is expected to increase, which can result in a numerical increase in CRS cases.  The potential for this phenomenon has led to a WHO recommendation that rubella not be added to the routine vaccination system until a country can maintain 80% vaccination coverage through a combination of routine immunization and campaigns Publication 2011; higher coverage is expected to significantly depress rubella prevalence and CRS risk. Local variation in rubella transmission rates, disparities in vaccination coverage, birth rates, and the age of first reproduction can significantly alter the expected threshold coverage above which introduction of rubella vaccination is expected to increase CRS incidence Lessler 2013 METCALF 2012. Dynamic models have been useful in assessing the potential risk of introducing rubella containing vaccine into routine programs and the degree to which different targeting strategies for supplemental campaigns can mitigate this risk Wesolowski 2016.