During the few weeks between the vaccination campaign and the coverage survey, there was a great deal of turn-over in the population. In one camp with a population of 12,900 during the campaign, 5500 persons left the camp and 7200 new persons entered, mostly directly from Kosovo. Most of the new children were unvaccinated, so the coverage, as measured by a cross-sectional survey, dropped substantially because vaccinated children left and unvaccinated children arrived. The administrative estimate is based on data collected at the time of the vaccination campaign, while the survey collected data were collected some weeks later. This example also illustrates the importance of vaccinating new arrivals to maintain a high population coverage.