Burkina Faso's immunization program has benefited regularly from national and international support. However, national immunization coverage has been irregular, decreasing from 34.7% in 1993 to 29.3% in 1998, and then increasing to 43.9% in 2003. Undoubtedly, a variety of factors contributed to this pattern. This study aims to identify both individual and systemic factors associated with complete vaccination in 1998 and 2003 and relate them to variations in national and international policies and strategies on vaccination of rural Burkinabé children aged 12-23 months.

Methods
Data from the 1998 and 2003 Demographic and Health Surveys and the Ministry of Health's 1997 and 2002 Statistical Yearbooks, as well as individual interviews with central and regional decision-makers and with field workers in Burkina's healthcare system, were used to carry out a multilevel study that included 805 children in 1998 and 1,360 children in 2003, aged 12-23 months, spread over 44 and 48 rural health districts respectively.

Results
In rural areas, complete vaccination coverage went from 25.9% in 1998 to 41.2% in 2003. District resources had no significant effect on coverage and the impact of education declined over time. The factors that continued to have the greatest impact on coverage rates were poverty, with its various dimensions, and the utilization of other healthcare services. However, these factors do not explain the persistent differences in complete vaccination between districts. In 2003, despite a trend toward district homogenization, differences between health districts still accounted for a 7.4% variance in complete vaccination.

Conclusion
Complete vaccination coverage of children is improving in a context of worsening poverty. Education no longer represents an advantage in relation to vaccination. Continuity from prenatal care to institutional delivery creates a loyalty to healthcare services and is the most significant and stable explanatory factor associated with complete vaccination of children. Healthcare service utilization is the result of a dynamic process of interaction between communities and the healthcare system; understanding this process is the key to understanding better the factors underlying the complete vaccination of children.