The scale of the delays varied dramatically by vaccine. Delayed doses accounted for 45.99 percent of DTaP-IPV/Hib doses, 40.69 percent of PCV13 doses, 47.15 percent of EV71 vaccine doses, and 62.13 percent of LLR vaccine doses. By contrast, only 2.40 percent of RotaTeq doses were delayed, a striking exception indicating that timeliness is not an inherent property of self-paid vaccination but varies with vaccine-specific factors such as dosing schedules and administration windows. The severity of delay was also considerable: 32.18 percent of all delayed doses were administered six months or more after the recommended window, meaning that a substantial share of children went without on-time protection for extended periods during their early years.

To move beyond simple delay counts, the research team applied machine learning and statistical modeling to the registry data. Gradient-boosted decision trees combined with SHAP value analysis identified the predictors of delay, while proportional-odds logistic regression assessed delay severity and negative binomial regression modeled the duration of delays across four phases of the COVID-19 pandemic. Vaccine type emerged as the strongest predictor of delay, followed by dose sequence, vaccination year, and city of vaccination. This hierarchy indicates that structural and product-level characteristics, rather than random variation, shape which doses slip past their recommended windows.

The pandemic period left a measurable imprint on the data. Both delay severity and delay-day rates remained higher during 2020 through 2024 than in the pre-pandemic period. As an observational registry analysis, the study documents this persistent elevation but does not establish the mechanisms behind it or the trajectory of timeliness after acute restrictions eased, and the authors do not report a completed recovery for these market-mediated vaccines.

First doses proved to be a particular weak point. The proportion of children receiving a delayed first dose ranged from 0.22 percent for RotaTeq to 15.71 percent for PCV13. Scenario-based estimations suggested that the potential reductions in timely protection would be greatest for EV71-associated disease and pneumococcal disease, the two vaccine-preventable conditions where delayed initiation was most consequential in the models. The authors conclude that immunization programs should monitor timeliness alongside coverage, since a fully covered cohort can still carry substantial gaps in on-time protection, and that recovery efforts should prioritize first doses and the vaccines carrying the highest delay burden.

Subject of Research: Dose-level timeliness of self-paid childhood vaccination in Chinese children aged 0 to 6 years

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

Article References: Chu, L., Chen, K., Fu, C., Zhou, Y., Wang, H., & Lin, L. K. (2026). Dose-level timeliness of self-paid vaccination in Chinese children aged 0–6 years: a registry-based analysis. International Journal for Equity in Health. https://doi.org/10.1186/s12939-026-03084-7

Image Credits: AI Generated

DOI: 10.1186/s12939-026-03084-7

Keywords: vaccination timeliness, self-paid vaccines, childhood immunization, China, COVID-19 pandemic, dose delays, PCV13, EV71 vaccine, registry-based analysis, first-dose delay, vaccine coverage, health equity