In many developing countries, Measles-Mumps-Rubella (MMR) vaccine is available through the private but not the public sectors, and there is no systematic rubella vaccination among adult women. In this paper, we extend previous modeling studies to demonstrate that in developing countries with a medium-high force of infection (200-400/1000 per year), current levels of private sector MMR coverage (<60%) would lead to increases in the incidence of Congenital Rubella Syndrome (CRS) both among unvaccinated individuals and the general population even when mixing between vaccinated and unvaccinated individuals is fairly minimal. Our findings highlight the need for countries to establish surveillance of trends in susceptibility to rubella and CRS incidence and perhaps introduce rubella vaccination among women of child-bearing age.