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Elimination of an infectious disease is often understood to mean the total absence of cases in a population. This situation can occur only if the entire population is immune as a result of either natural disease or vaccination. However, this costly and unrealistic scenario is not necessary to ensure elimination, more appropriately defined as a situation in which sustained transmission cannot occur and secondary spread from importations of disease will end naturally, without intervention. The authors describe the size and duration of outbreaks caused by imported infections after indigenous transmission has been eliminated. They show that the status of the elimination process can be monitored by assessing the proportion of cases imported and the distribution of outbreak sizes. Measles in Canada, the United States, and the United Kingdom provides a good example of the relevance of these criteria. Surveillance of …