Explanatory Factors of Vaccination Dropout Among Children Aged 12 to 23 Months in the Kikula Health Zone, Democratic Republic of Congo: Cross-Sectional Analytical Study

Background: Vaccination is among the most effective public health interventions to reduce childhood morbidity and mortality. Despite World Health Organization recommendations, global immunization coverage has declined in recent years, with the COVID-19 pandemic causing the largest sustained backslide in routine immunization in 3 decades. In the Democratic Republic of Congo (DRC), full immunization coverage remains below 50%, hindered by inequities, supply shortages, and financing delays. In the Kikula Health Zone, administrative reports suggest coverage exceeding 100%, yet independent surveys consistently reveal low completion and high dropout rates between Bacillus Calmette-Guérin (BCG) and measles vaccines. No previous study has specifically examined determinants of dropping out in this setting. Objective: This study assessed the prevalence and determinants of vaccination dropout between BCG and measles vaccines among children aged 12 to 23 months in the Kikula Health Zone, Likasi, DRC. Methods: An analytical cross-sectional survey was conducted from April 22 to May 22, 2025, using 3-stage cluster sampling to recruit 300 mother-child pairs. Vaccination status was verified using cards; for children without documentation, caregiver recall and health facility registers were used to minimize misclassification. Structured questionnaires captured sociodemographic data, child characteristics, maternal knowledge, perceptions of services, and health system access. Dropout was defined as receipt of BCG but not the measles vaccine. Bivariate associations were tested using chi-square tests, and multivariate logistic regression identified independent predictors, with robust SEs to account for clustering. Ethics approval was obtained from the University of Kinshasa School of Public Health. Results: Among 300 children, 115 (38.3%) had dropped out between BCG and measles vaccination, while 185 (61.7%) completed the schedule. Possession of a vaccination card was the strongest predictor: children without a card had 30-fold higher odds of dropout (adjusted odds ratio 30.9, 95% CI 11.6 82.0; P<.001). Other factors associated with dropout in bivariate analysis included shorter residence duration (≤5 y), lower maternal education, and nonuse of child health services, although these lost significance in multivariate models. Maternal knowledge gaps were notable: 169 (56.3%) did not know their child's vaccination status and 148 (49.3%) expressed fear of side effects. Service perceptions were generally positive (participants reporting good reception: n=294, 98%), but 108 (36%) experienced waiting times of 1 to 2 hours. The exclusion of undocumented children likely led to underestimation of dropout prevalence. Conclusions: Vaccination dropout between BCG and measles remains high in the Kikula Health Zone, driven primarily by lack of vaccination cards and maternal knowledge gaps. Administrative coverage data (>100%) mask substantial dropout, underscoring the need for improved documentation, maternal education, and targeted outreach. Programmatic implications include strengthening card management, deploying mobile vaccination units, and enhancing community reminders. Findings highlight the importance of addressing both demand and supply-side barriers to reduce dropout and improve equity in immunization coverage in the DRC.