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.