We grapple with such issues frequently while managing a WB E program for Healthy Davis Together (HDT), a multipronged pandemic-response initiative in Davis, CA. Since launching in September 2020, the program has grown to include in-house analysis of wastewater collected on a weekly, biweekly, or daily basis from 70 sites distributed across the City of Davis and the University of California, Davis (UC Davis) campus sewer systems and from the in fluent of their wastewater treatment plants.
We are glad that our wastewater data are informing local COVID-19 mitigation efforts. Results from wastewater collected from UC Davis dorm outflows are supporting the safe return of students to campus; results from wastewater collected from neighborhoods and broader city areas are helping public officials understand spatial changes in COVID-19 trends and react accordingly.
At the same time, launching and running a WB E campaign requires significant investments of time, money, labor, and expertise. Given that much information gleaned from wastewater is not directly actionable, and/or duplicates information from other sources, it is prudent to consider when these investments are worthwhile. Here, we offer insights based on our experience with HDT about when WB E makes sense and when constraints argue for spending scarce resources elsewhere.
The history of WB E has become a well-told story among practitioners (1). Although proposed as far back as the mid-1940s (2), WB E only began to gain traction as an epidemiological tool in the early 21st century. Applications of WB E in the 2000s and the 2010s were diverse—including monitoring use of pharmaceuticals (3) and illicit drugs (4), tracking flu prevalence (5), and, perhaps most notably, containing a polio outbreak (6)—but remained known to only a relatively small group of specialists.
In 2020, the COVID-19 pandemic catapulted WB E into mainstream attention. Rapid disease spread coupled with global shortages of clinical tests drove attention to early reports (7) demonstrating the utility of WB E for tracking COVID-19. The following months saw colleges, cities, and states alike incorporate WB E into pandemic response. There are now hundreds of WB E programs, comprising thousands of sites, tracking COVID-19 worldwide (8). Such programs can provide—and are providing—meaningful public health benefits. But it is important to recognize their limitations.
Individuals infected with SARS-CoV-2 typically begin excreting the virus several days before becoming symptomatic, and hence several days before they are likely to seek COVID-19 testing. WB E can therefore help public health officials proactively identify “hotspots” of disease emergence and spread (9). The value of WB E as a leading indicator of infection was heralded early in the pandemic, especially amid prolonged delays in access to and delivery of diagnostic testing results.
But as Olsen, and colleagues persuasively argue, WB E serves as a true early warning system only when background levels of COVID-19 are very low and clinical testing of the surveilled population is scarce or deficient (10). Otherwise, WB E can serve as an independent indicator of disease prevalence but not necessarily a leading indicator of outbreak potential. The extent to which sewage prevalence of SARS-CoV-2 leads to community infection also depends on physical characteristics of the sewer shed. Indeed, our comparisons of wastewater results with clinical results from HDT’s (widely accessible and widely used) asymptomatic-testing program show good agreement between the two datasets but no consistent lead of one indicator over the other (11).
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