Timeliness of childhood vaccination in Iran: Survival analysis of a national immunization survey, 2019.

OBJECTIVES

Despite high vaccination coverage in Iran, delays in administering age-appropriate vaccines may compromise overall protection. This study used survival analysis to examine the timeliness of vaccine delivery and its sociodemographic correlates.

METHODS

We analyzed the vaccination histories of 8248 children aged 24-35 months from the 2019 National Immunization Coverage Survey. Timeliness for the third pentavalent dose (Penta3) and the first measles-mumps-rubella dose (MMR1) was assessed via Kaplan-Meier curves and Cox proportional hazards models, with time-dependent effects where the proportionality assumption was violated. The Gini index was accounted for through stratification.

RESULTS

Although Penta3 coverage was high at 99.13%, only 72.23% of participants had received their vaccinations within the recommended time frame (P < 0.001). In contrast, the disparity between total coverage (99.04%) and valid coverage (94.36%) for MMR1 was notably less pronounced (P = 0.003). For Penta3, maternal literacy was associated with higher on-time vaccination (hazard ratio [HR] = 1.38; 95% confidence interval [CI] = 1.16-1.66), whereas a birth order ≥4 was associated with lower timeliness (HR = 0.72; 0.63-0.82). For MMR1, urban residence was associated with a lower baseline hazard of on-time vaccination (HR = 0.57; 0.49-0.67), alongside a modest increase in the time-dependent HR (TD-HR = 1.02; 1.01-1.03), suggesting partial convergence over time. Maternal literacy showed no strong main effect for MMR1 (HR = 0.94; 0.64-1.37), with a slight positive time trend (TD-HR = 1.01; 0.99-1.04).

CONCLUSION

Even with high coverage, gaps in on-time vaccination remain-particularly, among higher-parity children and in urban settings at baseline. The findings underscore the value of routinely reporting timeliness (not just total coverage) and of targeting context-specific barriers (e.g. for large families and urban clinics) to narrow disparities in on-time immunization.