Use of the Improvement Index to Evaluate Equitable COVID-19 Vaccine Allocation in the San Francisco Bay Area

Abstract

A social determinants of health index score, or Vaccine Equity Metric (VEM), was used to prioritize resources and address geographical disparities in California's vaccination coverage. We calculated the improvement index (II), or percent of the vaccination disparity gap closed, to evaluate the effects of this vaccination strategy in the SF Bay Area during the SARS-CoV-2 Delta variant surge. We conducted a cross-sectional study on SF Bay Area zip codes during the Delta surge (July 6-October 5, 2021). Data came from the California Immunization Registry and 2019 5-year American Community Survey. We used Spearman correlations to examine relationships between VEM category and vaccine coverage, and Kruskal-Wallis tests to compare vaccination II across VEM categories. We studied 248 zip codes in the SF Bay Area. Those with the lowest resources (VEM Level 1) had the highest absolute increase in vaccination coverage (14.3 vs. 5.4 percentage points in VEM Level 4); though a contribution was higher starting vaccination rates in Level 4 zip codes with the greatest resources. The ratio of vaccination coverage between the lowest and highest resourced zip codes increased from 0.79 to 0.9, suggesting reduced disparity. However, it is difficult to interpret given wide differences in n (Level 1 n=8 vs. Level 4 n=151). In contrast, the vaccination II accounts for each VEM category's baseline vaccination; all were statistically similar (grand mean 41.5%, p=0.367), implying comparable improvement across all zip codes. Using VEM to identify and prioritize resources to vulnerable communities contribute to equitable vaccine allocation in the SF Bay Area. Our study demonstrates an example of the II's advantages over conventional health equity metrics, such as absolute differences and relative effect measures, which can overestimate an intervention's impact.azz health disparity, San Francisco Bay Area, improvement index.