Age-Adjusted Vaccine Exposure Index for Cross- Country Evaluation in Childhood Immunizations: Hypothesis-Generating Findings

Current immunization metrics primarily focus on population-level coverage and disease prevention but lack standardized tools to assess vaccine exposure patterns at the individual level during early infancy for comparing national immunization schedules. Background/Objectives: This study develops a necessary, age-adjusted metric to quantify early-life vaccine exposure in infants under one year, specifically designed for post-implementation vaccination policy evaluation to support the optimization of national immunization schedules directed to enhance safety and maintain high vaccine coverage. Methods: We introduce the Vaccine Exposure Index (VEI), an age-adjusted measure that quantifies cumulative vaccine exposure in infants under 12 months. The VEI accounts for the number, intensity, and timing of vaccine doses, providing a standardized approach to evaluate how schedules distribute exposure in infants across countries. A lower VEI reflects reduced intensity of neonatal exposure to vaccines, while a higher VEI indicates more concentrated early-life exposure. The VEI serves as a novel surveillance tool to guide and improve immunization schedules. Results: To demonstrate its applicability and utility as a comparative tool, we calculated the VEI using national immunization schedules from a sample of fourteen advanced countries and two highly populated nations with varying vaccination practices. We also used available data on neurodevelopmental outcomes (specifically autism spectrum disorder rates, while acknowledging the hot and contradictory debate) to empirically test the VEI's potential as an indicator of scheduling intensity. Results suggest that countries with a higher intensity of early-life exposure (higher VEI) tend to have vaccination schedules associated with a higher average number of vaccines and doses, often including vaccination at birth and concentrated administration at 6 and 12 months. Conclusions: The VEI serves as a vital comparative tool for identifying immunization schedules with concentrated early-life exposure and evaluating their alignment with nations having balanced and less concentrated vaccination schedules by 12 months in infants. The proposed VEI index does not question the necessity of vaccines or advocate for reduced coverage; rather, it provides a structured framework for optimizing the timing and dose distribution to minimize potential risks while simultaneously maintaining high vaccine coverage and community protection. By enabling cross-country comparisons and highlighting successful models (e.g., those achieving high coverage with lower VEI scores), this index offers a path for health policy optimization, promoting developmentally sensitive scheduling, and guiding future research on long-term outcomes to build greater public trust and security in immunization programs. Hence, the study presents as an exploratory framework and hypothesisgenerating findings that could inform future research on optimizing vaccine timing while maintaining high coverage and safety.