Rubella (German measles) is no longer endemic in the United States. Because of its worldwide distribution, imported cases continue to occur, although these sporadic cases do not result in sustained transmission. Rubella is usually a minor illness in adults. Of major significance, however, is the high incidence of a constellation of congenital defects in children whose mothers are infected during early pregnancy. Typical anomalies caused by this congenital infection, known collectively as the congenital rubella syndrome (CRS), include hearing impairment, cardiac defects, cataracts, and developmental delay.
PATHOGENESIS AND EPIDEMIOLOGY
Rubella is a member of the Togaviridae family (a single serotype and the only member of the genus Rubivirus) with an approximately 10-kb (9762 nucleotides) single-stranded, positive-sense polyadenylated RNA genome and a lipid envelope (thus the Latin name of toga or cloak). The virus was first isolated in 1962 by Weller from the urine of his son. Current proposed nomenclature for wild-type and vaccine rubella viruses employs clades, genotypes, and a more precise sequence database. Rubella shares physicochemical properties with group A arboviruses. The virion is roughly spherical, 60 to 70 nm in diameter, with an icosahedral nucleocapsid composed of multiple copies of a single virus-specified structural capsid protein (C) that is covered by a lipid envelope in which 2 virus-specified structural glycoproteins (E1 and E2) are embedded. E1 appears to function in attachment, fusion, hemagglutination, and neutralization. In addition, there are 2 virus-specified nonstructural proteins (p90 and p150). The virus is thermolabile; inactivation is rapid at 37°C (98.6°F) and at room temperature.
Humans are the only natural host. Direct person-to-person airborne spread by infected nasopharyngeal secretion droplets appears to be the usual mode of transmission. The patient with subclinical infection is also a source of rubella virus. Patients are most contagious for a few days before and after the onset of rash, although virus may be present in pharyngeal secretions for as long as 1 week before and 2 weeks after the onset of rash. Infection acquired postnatally does not produce a chronic carrier state. Prolonged shedding occurs only in patients with CRS, which is characterized by chronic infection; infants may remain contagious for months after birth.
Although rubella occurs in all areas of the world, epidemiologic patterns vary from country to country. Mathematic modeling predicts that elimination of transmission requires approximately 90% immunity among children. Prior to widespread use of rubella vaccines in the United States, when 85% of the population was immune, rubella occurred primarily in children during the elementary school years. A small minority did not become infected until early adulthood. After vaccine introduction, there was a marked shift in epidemiology and a shift in age at which those who are susceptible became infected.
When rubella was endemic in the United States, its seasonal peak occurred in late winter and spring. Its greatest impact resulted from epidemics that occurred ...