Live, attenuated measles virus vaccine has been widely used in the United States since licensure in 1963, and approximately 25 million doses have been distributed. Extensive use of the vaccine has confirmed its safety and efficacy and nationwide immunization programs have had a profound effect on the incidence of measles, with the number of reported cases reaching an all-time low this year. Available evidence indicates that a single inoculation of currently licensed live virus preparations probably provides the same lasting immunity as that which follows natural measles infection.
Experience with inactivated (killed) measles virus vaccine has been less favorable. Early trials revealed certain disadvantages of inactivated virus as compared with live virus; multiple inoculations were required (three at monthly intervals), and the immunogenic response as well as the protective capacity proved to be less effective. During the past two years various reports have documented the occurrence of unusual and