Dose-level timeliness of self-paid vaccination in Chinese children aged 0–6 years: a registry-based analysis

Timely vaccination is essential, but dose-level delays remain poorly characterized. We assessed delays in self-paid childhood vaccination, identified predictors, and examined first-dose delay. We analyzed 7,946,943 dose records in three Chinese cities for five self-paid pediatric vaccines: DTaP-IPV/Hib, PCV13, EV71 vaccine, RotaTeq, and LLR vaccine. Delay was defined as vaccination after the recommended window and classified by severity. Gradient-boosted decision trees with SHAP identified predictors, proportional-odds logistic regression assessed severity, and negative binomial regression modeled duration across four pandemic phases. Scenario-based estimations were performed for first-dose delay. Delay was common but vaccine-specific. Delayed doses comprised 45.99% of DTaP-IPV/Hib, 40.69% of PCV13, 47.15% of EV71 vaccine, and 62.13% of LLR vaccine doses, versus 2.40% for RotaTeq; 32.18% of delayed doses were delayed by ≥ 6 months. Vaccine type was the strongest predictor, followed by dose sequence, vaccination year, and city. Delay severity and delay-day rates remained higher during 2020–2024 than in the pre-pandemic period. First-dose delay ranged from 0.22% (RotaTeq) to 15.71% (PCV13), and scenario-based estimates suggested potential reductions in timely protection were greatest for EV71-associated and pneumococcal diseases. Timeliness gaps in self-paid childhood vaccination were substantial and persistent across the COVID-19 period. These findings highlight the need to monitor timeliness alongside coverage in market-mediated immunization programs. Recovery efforts should prioritize first doses and vaccines with the highest delay burden.