Bias with respect to socioeconomic status: A closer look at zip code matching in a pneumococcal vaccine effectiveness study

Abstract

In 2010, 13-valent pneumococcal conjugate vaccine (PCV13) was introduced in the US for prevention of invasive pneumococcal disease in children. Individual-level socioeconomic status (SES) is a potential confounder of the estimated effectiveness of PCV13 and is often controlled for in observational studies using zip code as a proxy. We assessed the utility of zip code matching for control of SES in a post-licensure evaluation of the effectiveness of PCV13 (calculated as [1-matched odds ratio]*100). We used a directed acyclic graph to identify subsets of confounders and collected SES variables from birth certificates, geo-coding, a parent interview, and follow-up with medical providers. Cases tended to be more affluent than eligible controls (for example, 48.3% of cases had private insurance vs. 44.6% of eligible controls), but less affluent than enrolled controls (52.9% of whom had private insurance). Control of confounding subsets, however, did not result in a meaningful change in estimated vaccine effectiveness (original estimate: 85.1%, 95% CI 74.8–91.9%; adjusted estimate: 82.5%, 95% CI 65.6–91.1%). In the context of a post-licensure vaccine effectiveness study, zip code appears to be an adequate, though not perfect, proxy for individual SES.

Using Index of Concentration at the Extremes as Indicators of Structural Racism to Evaluate the Association with Preterm Birth and Infant Mortality—California, 2011–2012

“…Differences in the gradient of risk associated with ICE measures in the present study compared to previous studies are likely due to measuring [34,38,53]. We focused on zip codes as they represent larger local geographic units providing more robust estimates of racial and economic disparity [43,49,50]. Findings from the Public Health Disparities Geocoding Project suggest that zip codes provide instable socioeconomic estimates, in comparison to census tracts and blocks, and recommend using census tract as the unit of analysis for monitoring public data [42].…”

“…There are currently no studies that have assessed the utility of ICE measures in understanding poor health outcomes at the zip code level. Despite critiques about zip code level data [42,43], zip codes are a commonly used geographic unit to examine the relationship between neighborhood exposures and poor health outcomes [44][45][46][47][48], In comparison to census tracts, zip codes are slightly larger geographic units and can provide more stable estimates of neighborhood measures of racial and economic disparities [49,50]. Additionally, patterns of inequality may differ across geographic units warranting investigation to assess if social determinants should also be monitored at the zip code level [34][35][36].…”

Inequalities in Pediatric Fracture Care Timeline Based on Insurance Type

“…This method of using zip code-based median incomes is a robust, well-established, widely used metric to determine the income levels for medical studies. 15 16 17 18 This technique uses a large population, readily available data and zip codes and does not require notable economic investment that other techniques, such as tract-based measures, require. 15 , 16 …”

Generalisability of vaccine effectiveness estimates: an analysis of cases included in a postlicensure evaluation of 13-valent pneumococcal conjugate vaccine in the USA

“…Detailed information on eligible and enrolled controls has been previously published. 23 Vaccine providers for cases and controls were contacted and state or city immunisation registries were reviewed to obtain vaccine and medical histories. Additional details on enrolment procedures have been previously published and are summarised in the online supplementary material for the present study.…”

“…This analysis, therefore, focuses on representativeness of enrolled cases with respect to eligible cases and leaves questions concerning controls for other analyses. 23 …”