Measles is an almost invariable clinical experience of childhood, and is often a severe disease, frequently complicated by middle-ear infection or bronchopneumonia. In some countries it is a major cause of illness and death. Encephalitis occurs in approximately one of every 2000 reported cases; survivors often have permanent brain damage and mental retardation. Death, predominantly from respiratory and neurological causes, occurs in one of every 3000 reported measles cases. The risk of death is greater for infants and adults than for children and adolescents.
Contracting measles during pregnancy increases fetal risk. Most commonly, this risk involves premature labour and moderately increases rates of spontaneous abortion and of low birth weight.
Subacute sclerosing panencephalitis, a slow virus infection of the cental nervous system, is associated with measles virus. It is noteworthy that, in the United States of America, widespread use of measles vaccine has led to the virtual disappearance of this infection.
Immunization against measles has been of interest to WHO for many years, and especially since the Expanded Programme on Immunization was launched in 1974 with measles as one of the principal diseases against which it was directed. In support of the immunization programme, reference materials for anti-measles serum and measles vaccine virus have been established. The Requirements for Measles Vaccine (Live) were adopted by the WHO Expert Committee on Biological Standardization in 1966, with an Addendum in 1981 to include the accelerated test for stability. Since the original production of measles vaccine, the search for improved immunizing agents has continued.