Burundi is a small densely populated nation located in east-central Africa. It is divided into 24 health sectors. Vaccination against measles, targeted at children 9-23 months of age, was introduced in 1981 in Burundi as part of the World Health Organization's (WHO) Expanded Programme on Immunization (EPI). Between 1985 and 1988, as part of an initiative to improve child survival, UNICEF and other donors invested substantial resources, such as vaccines, syringes, refrigerators, transport, andfuel, in the Burundi EPI. In late 1988, the estimated vaccine coverage in Burundi was at its historical high. At the same time, Health Sector Muyinga, a sector in northeast Burundi, reported a dramatic increase in the number of measles cases diagnosed (Figure 2). This increase surprised the EPI central staff, since the EPI program in Health Sector Muyinga was highly regarded.