Interviewees are parents or grandparents of children aged between 6 to 59 months.

Interviewees are vaccination and health care personnel in each study site.

This is an observational study without any interventions.

This study will comprehensively collect vaccination programs for children under 6 years old in all 31 provinces of China (excluding Hong Kong, Macau, and Taiwan), covering both NIP and non-NIP vaccines that are suitable for children.

The economic value of optimized vaccination scheduling strategies with simultaneous vaccination and combination vaccines will be assessed using a cost-minimization and budget impact model with inputs collected in this study. The optimized vaccination scheduling strategie will be collected in this study through a literature review, a cross-sectional caregiver survey, a survey among vaccination staff, and an observational time and motion chart among the latter population.

To evaluate the acceptance and willingness of different vaccination strategies, the survey will collect caregivers' knowledge, attitudes, and preferences on optimized vaccine scheduling strategies including simultaneous vaccination and combination vaccines, as well as factors influencing their decisions , such as the vaccination status of siblings of other children and elderly family members over 60 years old. The primary outcome variable will be willingness to simultaneous vaccination and combined vaccines, assessed using a 5-point ordinal scale (from "very unwilling" to "very willing").

Using data collected from children's immunization records, a retrospective analysis will explore how previous simultaneous vaccination behaviors affect their vaccination rate. By comparing the vaccination rates at different ages between children receiving vaccines concomitantly vs. separately and children who uses combination vaccines vs. single vaccines, the study will evaluate whether the optimized vaccine scheduling strategies including simultaneous vaccination and combination vaccines have an impact on children's vaccination coverage.

A survey will be used to evaluate the attitudes and recommendation behaviors among vaccination staff towards optimized vaccine scheduling strategies including simultaneous vaccination and combination vaccines. The outcome variable will be recommendation behavior, assessed on a 5-point Likert scale ranging from "strongly recommend" to "strongly not recommend". To examine the perceived difficulty of vaccine scheduling, the survey will collect potential contributing factors: complexity of immunization schedules, inaccuracy of manual scheduling, limited implementation of simultaneous vaccination strategies, insufficient use of combination vaccines, high demand for catch-up vaccination, and lack of clear guidance for non-NIP vaccines.