When case reporting becomes untenable: Can sewer networks tell us where COVID-19 transmission occurs?

Abstract

Monitoring SARS-CoV-2 in wastewater is a valuable approach to track COVID-19 transmission. Designing wastewater surveillance (WWS) with representative sampling sites and quantifiable results requires knowledge of the sewerage system and virus fate and transport. We developed a multi-level WWS system to track COVID-19 in Atlanta using an adaptive nested sampling strategy. From March 2021 to April 2022, 868 wastewater samples were collected from influent lines to wastewater treatment facilities and upstream community manholes. Variations in SARS-CoV-2 concentrations in influent line samples preceded similar variations in numbers of reported COVID-19 cases in the corresponding catchment areas. Community sites under nested sampling represented mutually-exclusive catchment areas. Community sites with high SARS-CoV-2 detection rates in wastewater covered high COVID-19 incidence areas, and adaptive sampling enabled identification and tracing of COVID-19 hotspots. This study demonstrates how a well-designed WWS provides actionable information including early warning of surges in cases and identification of disease hotspots.

Keywords: Adaptive sampling; COVID-19; Community level; Hotspot; Sampling design; Wastewater surveillance.

Graphical abstract

Fig. 1

(a) Estimated catchment areas of nine influent lines sites in the city of Atlanta; The catchment area of Old Winn Dixie, which is very small, may not be visible. (b) locations for community sampling sites and school sampling sites.

Fig. 2

Indegree and outdegree of different flow network topologies.

Fig. 3

Illustration of adaptive sampling process in a simulation study. Each node is a manhole in the sewer network. For the risk area subfigure, the size of nodes represents the number of shedders that contribute SARS-CoV-2 into wastewater, as an indicator of COVID-19 infection risk. For subfigures update 0–7, manholes shown in black are on the longest line in the sewer network, defined as the main trunk (main stream). Manholes shown in yellow are covered by the selected sampling sites, while manholes shown in gray are not covered. Red manholes represent sites with positive wastewater samples (detection of SARS-CoV-2). Blue manholes represent sites where the wastewater samples were negative. The sizes of nodes represent the weights of sites. The larger the weight, the higher estimated risk of COVID-19 transmission. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Two sewer sub-networks (a) within the center of the city and (b) in the periphery of the city.

Fig. 5

(a) Geographical distribution of manholes with large indegree on the sewer network; (b) Geographical distribution of manholes with large outdegree on the sewer network; (c) Areas with combined sewers in the city of Atlanta.

Fig. 6

Temporal trends of the concentrations of SARS-CoV-2 RNA in wastewater from nine influent line sites and the reported case numbers during March 20, 2021–May 8, 2022. Solid purple symbols represent positive samples and open purple symbols show negative samples at the detection limit. The black line is the LOESS line for concentration of SARS-CoV-2 RNA, and the gray band represent the 95% confidence interval. The red line is the number of reported cases in Fulton County, Georgia. The yellow line is the number of reported cases in the catchment area of the influent site. Throughout the study period, no case was reported in the very small catchment area of Old Winn Dixie. The graph caption shows the Spearman correlation between the smoothed concentrations of SARS-CoV-2 RNA in wastewater and the number of reported cases in Fulton County. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

SARS-CoV-2 wastewater surveillance results for community sites and reported case numbers in Fulton County between March 20, 2021–April 15, 2022. The high reported case numbers in April 2022 were caused by a data dump (i.e., delayed reporting), and did not represent a surge in cases during that month. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8

SARS-CoV-2 wastewater surveillance results for community sites and reported COVID-19 case numbers in the catchment area for the Phillip Lee sampling cluster between September 2021–May 2022. Subfigure (a) shows the catchment areas of each community site nested within the overall catchment of the influent line site (in gray). The black lines represent the sewer network lines. Subfigure (b) shows the weekly wastewater surveillance results (RT-PCR detection of SARS-CoV-2 RNA). Subfigure (c) shows the heatmap of reported COVID-19 cases between September 1st 2021–May 8th 2022 within the influent catchment area. Subfigure (d) shows the epidemic curves of COVID-19 within each community site catchment area and NA represents all the cases in the catchment area of the influent line site that are not in the catchment area of a specific community site. PWT, PT, BEM, Cha, Wal, and Lar represent Peyton Woods Trail, Plainville Trail, Benjamin E Mays High School, Chatham Ave, Walmart, and Larchwood respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)