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Background: Use of combination vaccines has been associated with improved coverage rates, but their effect on timeliness remains to be explored. This study assessed the effect of diphtheria-tetanus-acellular pertussis/hepatitis B/inactivated polio vaccine (DTaP/HepB/IPV) on the timeliness of vaccine administration., Methods: This retrospective cohort study used administrative claims data from the Georgia Medicaid program. Children with 24 months of continuous enrollment and at least 4 vaccine-related office visits were stratified into 2 cohorts: those with at least 3 DTaP/HepB/IPV doses (DTaP/HepB/IPV cohort) and those with at least 3 doses of DTaP but no doses of DTaP/HepB/IPV (reference cohort). Children who received any dose of HepB/Hib were excluded to isolate the effect of the study vaccine. Timeliness was measured as the percentage of children who received their vaccines on time and the cumulative days undervaccinated., Results: There were 2880 children in the DTaP/HepB/IPV cohort and 2672 in the reference cohort. After controlling for covariates, receipt of DTaP/HepB/IPV was associated with significantly improved timeliness for 3 doses of DTaP (on-time rates: 66.3% vs. 60.8%, P < 0.0001; cumulative days undervaccinated: 29.5 vs. 70.4 days, P < 0.0001). Significantly improved timeliness was also observed in the DTaP/HepB/IPV cohort for IPV, HepB, Hib, 4 DTaPs, and the combination series assessed (P < 0.001 for all comparisons)., Conclusions: Use of DTaP/HepB/IPV in this Medicaid population was associated with improved on-time vaccination and fewer undervaccinated days. These findings, along with previous research associating combination vaccines with improved coverage rates, provide quantitative data to support the ACIP, AAP, and AAFP preference for combination vaccines.