Prevalence and Factors Associated with Incomplete Vaccination Among Children Under Five Years in N’Djamena-South Health District, Chad, 2025: A Community-Based Case-Control Study

Abstract Background Childhood vaccination is one of the most cost-effective public health interventions, yet coverage remains suboptimal in sub-Saharan Africa. Chad has one of the region’s lowest vaccination rates, with many children unprotected against vaccine-preventable diseases. We aimed to determine the prevalence and factors associated with incomplete vaccination among children under five years in N’Djamena-South Health District, Chad. Methods We conducted a case-control study nested within a community-based cross-sectional survey from February to June 2025. Using stratified two-stage random cluster sampling, we enrolled 1,132 children aged 0–59 months from 13 randomly selected health responsibility zones. Cases were incompletely vaccinated children (missing ≥ 1 age-appropriate vaccine dose); controls were fully vaccinated children with documented evidence. Data were collected through face-to-face interviews with parents/guardians and vaccination card review. Multivariable logistic regression identified factors associated with incomplete vaccination, reporting adjusted odds ratios (aOR) with 95% confidence intervals (CI). Results Among 1,132 children, 388 (34.3%; 95% CI: 29.0–39.9%) were incompletely vaccinated. BCG coverage was 93.2%, declining progressively to 65.7% for Penta3 and booster doses, indicating substantial dropout. Although vaccination services were reportedly available (98.8%), free (99.5%), and within 5 km for most households (95.3%), 30.7% of parents cited occupational constraints as the main barrier. Low parental education (aOR = 193.32; 95% CI: 77.15–479.42; p < 0.001) and low monthly income (< 100,000 FCFA vs. ≥100,000 FCFA; aOR = 2.83; 95% CI: 1.12–7.15; p = 0.027) were strongly associated with incomplete vaccination. Age 0–11 months was protective (aOR = 0.54; p < 0.001). Child sex showed no association. Conclusions One in three children under five in N’Djamena-South is incompletely vaccinated, driven primarily by low parental education and low income, despite good physical access to vaccination services. Interventions should address socioeconomic barriers through targeted health education, flexible vaccination schedules, and support for indirect costs.