Defining and assessing measles elimination goals.

Although 3 of 6 World Health Organization regions have established measles elimination targets, measles elimination goals are not well defined. In general, disease elimination has been defined as the reduction of incidence in a population to zero. However, measles is so contagious that zero incidence is difficult to achieve and sustain because the risk of imported measles remains while measles is endemic in any country. Also, imported cases will occasionally result in short chains of indigenous transmission unless a country achieves 100% immunity. Therefore, the United States currently uses the absence of endemic measles (i.e., no indigenous chains of transmission persisting for >or=1 year) as the programmatic goal for measles elimination. To document the absence of endemic measles in the United States, we compiled information on the epidemiology of measles, genotype distribution, population immunity, and adequacy of measles surveillance. A panel of experts, convened in March 2000 to review the data, concluded that measles is no longer endemic in the United States.