Has surveillance been adequate to detect endemic measles in the United States?

Evidence that endemic measles has been eliminated in the United States rests on the performance of the surveillance system. Information from national surveillance data allows us to evaluate the adequacy of national surveillance to detect the circulation of endemic measles. Sources of data include measles report dates, international importation status, and the size of chains of measles transmission. The proportion of chains of measles transmission that can be epidemiologically linked to international importations is high (62%), as would be expected if measles is no longer circulating; the number of imported cases, although lower than estimated expected values, is within a reasonable range of expectation. National surveillance detects even small outbreaks, so larger outbreaks that are the marker for endemic transmission would almost certainly be detected. Few unreported cases of measles are detected when health departments conduct careful investigations in response to reports of an index case. Surveillance appears to be adequate to support the contention that measles is no longer endemic in the United States.