Socioeconomic Factors Explain Racial Disparities in Invasive Community-Associated Methicillin-Resistant Staphylococcus aureus Disease Rates

Abstract

Background: Invasive community-associated methicillin-resistant Staphylococcus aureus (MRSA) incidence in the United States is higher among black persons than white persons. We explored the extent to which socioeconomic factors might explain this racial disparity.

Methods: A retrospective cohort was based on the Centers for Disease Control and Prevention's Emerging Infections Program surveillance data for invasive community-associated MRSA cases (isolated from a normally sterile site of an outpatient or on hospital admission day ≤3 in a patient without specified major healthcare exposures) from 2009 to 2011 in 33 counties of 9 states. We used generalized estimating equations to determine census tract-level factors associated with differences in MRSA incidence and inverse odds ratio-weighted mediation analysis to determine the proportion of racial disparity mediated by socioeconomic factors.

Results: Annual invasive community-associated MRSA incidence was 4.59 per 100000 among whites and 7.60 per 100000 among blacks (rate ratio [RR], 1.66; 95% confidence interval [CI], 1.52-1.80). In the mediation analysis, after accounting for census tract-level measures of federally designated medically underserved areas, education, income, housing value, and rural status, 91% of the original racial disparity was explained; no significant association of black race with community-associated MRSA remained (RR, 1.05; 95% CI, .92-1.20).

Conclusions: The racial disparity in invasive community-associated MRSA rates was largely explained by socioeconomic factors. The specific factors that underlie the association between census tract-level socioeconomic measures and MRSA incidence, which may include modifiable social (eg, poverty, crowding) and biological factors (not explored in this analysis), should be elucidated to define strategies for reducing racial disparities in community-associated MRSA rates.

Keywords: antibiotic resistance; methicillin-resistant Staphylococcus aureus; racial disparities; social determinants of health.

Figure 1.

Results of geocoding of community-associated methicillin-resistant Staphylococcus aureus (MRSA) cases.

Figure 2.

Methicillin-resistant Staphylococcus aureus (MRSA) incidence by race, stratified by quartiles of census tracts related to specific income, housing, education, and health variables. Incidences are displayed in increasing quartiles of census tracts, per 100000 persons for white (no fill) and black (solid fill) persons. The adjusted rate ratio (aRR) and 95% confidence interval (CI) of MRSA in black persons (reference group for aRR: rate in white persons) considering stratification is shown in each graph except when marked by (*) to indicate heterogeneity in RRs across quartiles (ie, interaction between race and the census tract measure). Refer to Table 1 for a description of individual variables.

Figure 3.

Results from mediation analysis to estimate the extent to which socioeconomic factors explain racial variation in invasive community-associated methicillin-resistant Staphylococcus aureus (MRSA) incidence. Diagram depicts the estimated rate ratio for the total effect of black race on invasive community-associated MRSA incidence (corresponding to unadjusted rate ratio and shown with the thick black arrrow) as well as a potential pathway in which race affects invasive community-associated MRSA incidence through socioeconomic factors (indirect effect of race) or independently (direct effect) as shown in dashed lines/arrows. Socioeconomic factors accounted for in the mediation analysis were proportion of expensive homes in a census tract, proportion of persons with high education, proportion of low-income households, proportion of persons living in a rural area, and being a medically underserved area.