Rubella has a worldwide distribution. It usually occurs in a seasonal pattern (i.e. in temperate zones during the late winter and spring), with epidemics every 5-9 years. However, the extent and periodicity of rubella epidemics is highly variable in both developed and developing countries. The reaosns for this are not known. Before the introduction of large-scale rubella vaccination, the average age at which children were infected varied between 6-12 years in industrialized areas and 2-8 years in urban areas of developing countries. The extent of susceptibility in women of childbearing age varies considerably, with study data ranging from <5% in Kuwait to 60% in rural Panama, mainly reflecting epidemiological and socioeconomic differences between the study populations. The highest risk of CRS is found in countries with high susceptibility rates among women of childbearing age. Although low susceptibility rates have been reported in studies of selected populations within some countries, these may reflect local variations, and extrapolating from such studies could mask a significant national benefit from the introduction of rubella vaccination.