Rubella virus (RUBV) usually causes a mild exanthematous disease that is frequently accompanied by adenopathy and occasionally by arthralgia. Complications of this infection are rare and include encephalopathy and thrombocytopenia. However, the most severe consequence of this virus is its teratogenicity. It can cause congenital rubella syndrome (CRS) when it occurs in pregnant women, particularly during the first trimester of pregnancy (10).The direct detection of RUBV RNA in clinical specimens, in addition to the detection of RUBV-specific immunoglobulin M, is a critical factor in the early laboratory diagnosis of recent or congenital infection (18,27). Currently, the European region of the World Health Organization (WHO) aims to eliminate not only measles but also rubella and to reduce the incidence of CRS to less than one case per 100,000 live births by 2010 (38, 39). For this purpose, epidemiological surveillance based on the laboratory diagnosis of each suspected case and the characterization of the genotype of the circulating strains are included in the WHO's recommendations. In the most recent WHO update, the standard nomenclature for the classification and designation of wild-type RUBV strains recognizes nine definitive and four provisional genotypes (40), expanding the nomenclature established in 2005 (37), which was based on 739 nucleotides (nt) (nt 8731 to 9469) from the E1 gene sequence. This sequence encodes amino acids (aa) 159 to 404 (of the 481 aa) of the E1 glycoprotein. Although our knowledge of the geographic distribution of RUBV genotypes has grown substantially since 2003, the genotypes present in many countries and regions remain unknown (9), even though rubella is still recognized as a globally important disease in a general public health context (41). RUBV is considered monotypic with cross-neutralization among different genotypes.In Spain, monovalent RUBV vaccine was introduced in the late 1970s, when it was administered in schools to 11-year-old girls (1). In 1981, one dose of the measles-mumps-rubella combined vaccine was introduced in the regular immunization schedule at the age of 15 months for all children. In 1996, a second dose at 11 years of age was introduced (5). In 1999, this second dose was given to 4-year-old children (3). Currently, the seroprevalence of RUBV in the community of Madrid exceeds 95% in all age groups and reaches 98.6% among women of childbearing age (16 to 45 years) (4). Nevertheless, the pattern is very different in other regions around the world and RUBV infection remains endemic in many areas, such as Latin America (15). The rubella vaccine was only introduced in Latin American countries in the late 1990s, so that many adult immigrants to Spain are not immunized. These circumstances led to a small outbreak in Madrid in 2003 (31) and a larger one in 2004 and 2005 (2, 27) among Latin American immigrants. The main aim of this study was to characterize the RUBV strain involved in the latter outbreak, which represents the first data concerning RUBV genotypes i...