Addressing gaps in surveillance: Use of emergency department accessibility and social vulnerability to identify potential wastewater surveillance sites in North Carolina

Background: The North Carolina Division of Public Health relies on multiple complementary surveillance systems, including emergency department (ED) and wastewater surveillance, to track disease trends across the state. Despite having comprehensive statewide ED coverage, several areas have limited access to ED facilities and concurrent higher social vulnerability. In this analysis, we demonstrate complementarity between North Carolina ED and wastewater surveillance and assess geographic gaps in coverage to identify potential new wastewater monitoring sites in low-coverage, high social vulnerability areas. Methods: Weekly wastewater SARS-CoV-2 viral concentrations and percentage of COVID-like Illness (CLI) ED visits across current wastewater sites geographically overlapping with hospitals, were compared using a Spearman’s rank-order correlation analysis for the period of November 2021 to August 2024. A drive-time analysis was conducted for 133 hospitals to identify areas within a 20–60-minute commute time to any ED facility. Out of a statewide sewershed polygon dataset, sewersheds serving a population greater than 1200 (n = 220) were selected for further investigation. ED service areas were then examined for overlap with sewersheds, and Social Vulnerability Index (SVI) rankings were compared for sites with a majority geographic overlap (>=50%). Results: Of the 28 overlapping hospitals, 26 show a moderate to strong correlation (rho = 0.303–0.718) with their respective sewersheds, demonstrating complementarity between the two systems. Of the 220 potential sewersheds examined, 40 overlapped with the 20–60-minute drive times to EDs. Of these, 26 wastewater sites had an overall SVI greater than 0.5 and were identified as priority additions to North Carolina’s wastewater surveillance network, including 13 with an overall SVI greater than 0.75. Conclusion: We used a novel geospatial approach to identify 26 priority wastewater sites with moderate to high social vulnerability and low ED accessibility. Additionally, this work further supports complementarity between ED and wastewater surveillance systems.