Want to Test More People for Covid-19? Look At the Sewage

There are two key challenges facing public health officials dealing with the coronavirus: a shortage of test kits and the fact that as many as 20% of people with Covid-19 could be asymptomatic. These two problems demand a new approach to testing—one that shifts from testing individuals to testing populations. Sewage surveillance serves this precise need. This data collection strategy can quickly provide information on viral levels circulating in a community at a small fraction of the cost of current testing, since wastewater testing can represent the exposure of hundreds to thousands of people.  

When analyzed alongside existing local data sources, wastewater data also can inform policy decisions and actions to manage the Covid-19 pandemic. A similar strategy was used to measure the opioid epidemic in Montana, and can serve as a roadmap for how state and local officials can use this innovative approach for the current crisis.

Wastewater-based epidemiology in Montana

In 2019, Dr. Deborah Keil at Montana State University led a 10-week study to assess opioid and illicit drug use at two Montana wastewater treatment plants. Mathematica, a policy research firm, partnered with Montana State University to conduct an analysis that compared drug use detected in wastewater with data provided by local public health and public safety partners on pharmacy prescriptions, law enforcement drug seizures and overdose calls to emergency medical services. That analysis found that wastewater testing can provide an early warning for health threats, fill in knowledge gaps and evaluate the impact of official government actions.

Providing an early warning for health threats

Often, public health threats come with little to no warning. Sewage surveillance might be able to change that. In the Montana study, we found that when wastewater testing estimated an increased level of drug use that more overdose calls surfaced within two weeks (See Figure 1), indicating the potential for local officials to better predict negative public health outcomes and to be more proactive in their response.

Globally, wastewater testing has provided an early warning for infectious disease epidemics in the past. Existing sewage surveillance programs in India and Israel provided critical early detection of the polio virus from 2010 to 2014, enabling officials to rapidly mobilize and intensify vaccine campaigns.

For Covid-19, which is shed in feces, likely in a noninfectious form, preliminary research in the Netherlands confirmed that the virus could be detected early in untreated wastewater, even when confirmed case counts were low.

Filling in knowledge gaps

Public health officials sometimes lack all the necessary information about public health threats. In Montana, we used data triangulation, which takes multiple data sources to understand a trend to measure the black market for illicit drugs. By overlaying trends in doses of a drug in filled prescriptions with trends in doses excreted into the wastewater, we saw that wastewater data suggested much higher use of methamphetamines than the pharmacy data, potentially reflecting the scale of black-market meth use.

A key strength of wastewater data is broad population coverage. In Montana, the wastewater plants we worked with served one-third of the rural county’s population and 42% of the urban county’s population. Nationally, coverage extends to 71% of Americans.

For Covid-19, wastewater testing could fill a critical gap in our knowledge about population exposure to the virus, helping us assess the unmeasured threat of Covid-19 beyond the small fraction of clinically confirmed cases.

Evaluating the impact of official policies and actions

Finally, our Montana study revealed how wastewater data can be combined with existing local data to measure the impact of official actions—in this case, policing—to reduce drug use. By superimposing the timing of large methamphetamine or amphetamine seizures over graphs showing trends in community use of those drugs, we could clearly see the effect that each major seizure had on reducing drug use (See Figure 2).

For Covid-19, officials can use wastewater data to measure the impact of policies implemented to reduce viral spread. They can see whether social distancing successfully reduces circulation of the virus, and gauge when to lift restrictions without endangering public health.

Widening the Lens

The most significant benefit of wastewater testing is its ability to flexibly measure and monitor a range of population health indicators circulating through a population. Critically, the same sampling infrastructure used for drug epidemics can be used to manage an infectious disease pandemic. That infrastructure already exists, as employees at 15,000 wastewater treatment plants across the country are routinely collecting wastewater samples to measure toxins regulated by the U.S. Environmental Protection Agency.

For Covid-19, early pilot studies in Massachusetts and Montana have confirmed that the virus is detectable in untreated wastewater samples, and officials at more than 170 sites across 37 states are now submitting wastewater samples for testing. In other words, the workflows for sampling already exist, and testing is available through a handful of commercial and academic labs.

To scale the methodology for epidemic management, we need coordination and the equivalent of a supply chain for wastewater testing. If we can connect the sampling to the testing, and the testing results to other data (such as data Mathematica is curating on social vulnerability, provider capacity, patient needs and policy actions), we can facilitate the types of comparative analyses that provide practical insights for policymaking.

Our lack of capacity to individually test even 2% of the U.S. population right now is impeding officials’ ability to manage Covid-19. To reliably gauge whether viral levels are increasing or decreasing in a community, we need to pair individual testing with wastewater testing. Although wastewater data does not allow for contact tracing, since the data cannot reveal who in a community is infected, its broad coverage will yield a more complete picture of disease prevalence and trends. If state and local officials invest in this rich, near-real-time data source now, they can give themselves a better chance at preventing second-wave infections and averting future epidemics.