Quantifying Inequalities in Childhood Immunization Using Summary Measures of Health Inequality: An Application of WHO...

Background: Monitoring immunization inequalities is crucial for achieving equity in vaccine coverage. Summary measures of health inequality provide a single numerical expression of immunization inequality. However, the impact of different summary measures on conclusions about immunization inequalities has not been thoroughly studied. Methods: We used disaggregated data from household surveys conducted in 92 low- and middle-income countries between 2013 and 2022. Inequality was assessed for two indicators of childhood immunization coverage [three doses of combined diphtheria, tetanus, and pertussis (DTP) vaccine and non-receipt of DTP vaccine or "zero-dose"] across three dimensions of inequality (place of residence, economic status, and subnational region). We calculated 16 summary measures of health inequality and compared the results. Results: These measures of inequality showed more similarities than differences, but the choice of measure can affect inequality assessment. Absolute and relative measures sometimes produced differing results, showing the importance of using both types of measures when assessing immunization inequality. Outliers influenced differences and ratios, but the effect of outlier estimates was moderated through the use of complex measures, which consider all subgroups and their population sizes. The choice of appropriate complex measure depends on the audience, interpretation, and outlier sensitivity. Conclusions: Summary measures are useful for assessing changes in inequality over time and making comparisons across different geographical areas and vaccines, but assumptions and value judgements made when selecting summary measures of inequality should be made explicit in research.