The perfect pair — wastewater and clinical tests for COVID-19

We know that COVID-19 testing still matters. But which is better: Wastewater-based or clinical COVID-19 test data? The answer is easy — Both.

To understand why, it’s important to first understand differences in how samples are collected for each type of test.

Wastewater testing collects information from groups of people. Virus is shed in the stool of infected individuals among thousands of other stool samples that are flushed down the toilet daily, mixed together in the sewer system, and collected downstream in a vial in aggregate.

Clinical testing collects information from individuals. Virus is shed in mucus and is collected on a nasal swab.

These differences translate to different benefits for each test.

Wastewater-based testing can act as a community early warning system

Wastewater-based tests often act as a leading indicator of an outbreak in a community for three key reasons:

1. Biology: Depending on the disease, virus can be shed in stool before individuals begin to experience symptoms, and individuals often don’t seek clinical tests until they start to feel sick.
2. Logistics: It is not feasible or practical to conduct a clinical test every day on every individual in a community. Wastewater-based testing can be conducted daily and, in most cases, can be incorporated into routine wastewater operational procedures.
3. Equity: Wastewater-based tests better represent the community as a whole. Each test represents every individual in the community that has used a toilet connected to a sewer, independent of where they live, whether they experience symptoms, or whether they have the time, money or access to clinical testing.

Clinical testing addresses individuals

Clinical tests diagnose disease in individuals that seek medical care. A diagnosis leads to a personalized treatment plan and, ideally, a full recovery. And clinical testing provides quarantine guidance and enables contact tracing, which helps break transmission chains. Clinical tests are a mission-critical tool for diagnosing individuals and stopping the spread of disease.

What does it all mean?

Wastewater-based and clinical tests have a symbiotic relationship. Both data sources have their limitations, but when used together they provide independent confirmation of what is happening in a community.

Wastewater-based tests can identify hidden infection hotspots in communities, which can then focus clinical testing efforts and community outreach programs and make both more efficient and impactful.

As infection rates go down, wastewater-based testing can be used to monitor community health proactively on an ongoing basis.

A real-world example

Imagine a community composed of four sewersheds that each represent 10,000 individuals. Now imagine that each sewershed is represented by one wastewater-based test, conducted daily. So, four sewersheds, four sampling locations, four daily tests, 10,000 people per test per day, 40,000 total people tested per day.

It would be nearly impossible to conduct clinical tests on all of the individuals in this community on a daily basis. But, with frequent wastewater testing, communities can understand whether or not disease is present and whether the incidence of disease is increasing or decreasing over time.

In our sewershed scenario, imagine that a community suddenly detects a sharp increase of virus in wastewater from Sewershed 1, a moderate increase in Sewershed 4 and no change in Sewersheds 2 and 3.

Empowered with this information, community leaders and public health officials can respond in a proactive, efficient and focused way. In Sewershed 1, leaders and officials may decide to reserve additional hospital beds, ramp up clinical testing capacity and focus vaccination efforts. In Sewershed 4, they may begin preparations to do the same if the upward trend continues. And, they’ll likely ramp up public health campaigns across all four sewersheds and share these results, which would help emphasize the need to stay vigilant about disease prevention measures that may already be in place.

The same is true in reverse. By monitoring reductions in the incidence of disease, leaders and officials can make better decisions about when and how to reopen schools and businesses and when to relax mask-wearing policies.

Somewhat counterintuitively, the need for wastewater-based testing increases as disease incidence decreases and as public health improves. As vaccinations and declining cases provide a sense of security, individuals may let their guard down, and may not choose to seek out clinical tests when experiencing mild symptoms. Results from wastewater-based testing, on the other hand, aren’t affected by clinical test seeking behavior, just by who uses a toilet connected to a sewer.

Looking forward

We need both types of tests to combat the spread of COVID-19, identify and treat those that are sick, and reopen communities in a safe and data-informed way.

Earlier, we imagined a community with four sewersheds in the COVID-19 pandemic. Now imagine if we tested wastewater in every community across the United States on an ongoing basis. Would the next pandemic be as bad?