Chopsticks in the nose or throat: Tests are not the only way to find out how widespread the coronavirus is. From the point of view of experts, sewage treatment plants could also serve as an early warning. Should this opportunity be used more?

Relatively low costs, little effort and a real-time picture of the situation of the pandemic: medical officers recently advocated expanding wastewater analyzes to include traces of corona. “It would be ideal if all municipalities participated,” said the chairman of the Federal Association of Physicians in the Public Health Service, Johannes Nießen, of the Funke media group. So far, a pilot project has been running in Germany with 20 locations, including Berlin. Questions and answers on this.

How do such investigations work?

First, wastewater samples are taken in the sewage treatment plant. “Small bottles with 200 milliliters twice a week are enough,” said Emanuel Wyler, who has been carrying out analyzes of virus variants at the Max Delbrück Center for Molecular Medicine in Berlin since the beginning of 2021. The wastewater contains very diluted tiny virus components that some infected people excrete when brushing their teeth or using the toilet. “We extract the viruses from the sample and then carry out a PCR test, as is also known from swabs in the nose or throat.” The sample is not only examined for Sars-Cov-2, but also for harmless but widespread plant viruses for comparability.

What can sewage tests do to detect the coronavirus?

The advantages are the lead time compared to the official pandemic data and the independence from the number of (PCR) tests carried out. In short, while only some people get tested, everyone needs to go to the bathroom. According to experts, with the waste water you are closer to the actual infections because infected people excrete viruses even before the onset of illness. The number of reports, on the other hand, lags behind the actual development because time elapses from the infection to the outbreak of the disease to the PCR result and the report to the Robert Koch Institute (RKI).

The information on the lead time compared to the RKI curves varies: while the head of the laboratory at Berliner Wasserbetriebe spoke of seven days in an interview, for example, the city of Cologne gave a lead of four to ten days in a statement. Virus variants can also be detected with additional tests. According to experts, it is also technically possible to take a close look at places of particular interest such as large companies or airports.

“Wastewater monitoring can represent developments over time quite well and may also help to identify hotspots, depending on how small-scale it is done,” says the Bremen epidemiologist Hajo Zeeb. In this respect, it is a “valuable addition” – also because, in the best case, it provides information quickly.

What does such an approach cost?

According to the Berliner Wasserbetriebe, a test for Sars-CoV-2 detection costs around 300 euros. Determining the variant costs another 200 euros. “We have purchased a digital PCR device and hired a microbiologist,” said a spokesman. Part of the analysis will be outsourced to an external laboratory. “Many smaller water suppliers in particular without their own laboratories would certainly have to go this way,” said the spokesman.

The German Association for Water Management, Wastewater and Waste (DWA) speaks of annual costs of 14 million euros if the 235 largest sewage treatment plants in Germany were included – this covers around half of the population. Researcher Wyler doesn’t need to have samples from every community either. “That would be far too expensive.” Rather, samples from the 100 largest sewage treatment plants in Germany are sufficient. “If you took samples there twice a week, you would end up with a weekly cost of several tens of thousands of euros.”

Is the sewage test worth it?

That should also be one of the questions in the ongoing pilot project. According to the description, this is intended to determine, among other things, how the concepts developed so far for local contexts can be transferred to other regions. It is scheduled to run until March 2023.

For Wyler, the big question is what to do with the knowledge that a location is erupting or the next wave is looming. In the event of a certain development, for example, will more people be sent to work from home again? Do you impose a mask requirement or does a hospital issue a vacation ban as a precaution? “Politics must answer that.”

Wyler also points out that wastewater values ​​do not provide any information about the severity of the disease and only show trends in the infection process. It is not possible to draw conclusions about the number of people currently infected in the population. According to researchers, a certain amount of infection is also required to be able to detect the virus in wastewater.

What role models are there?

In numerous countries, wastewater is examined for Sars-CoV-2 in smaller projects or in the course of larger surveys. In neighboring countries such as Austria and the Netherlands, corona dashboards on the Internet show the results. Regional information is also available. The idea itself is not new: wastewater analysis for polio has been carried out in some countries for many years. Knowledge can also be gained in this way about drug use in cities. In addition to corona, scientists are also interested in other pathogens such as influenza and antibiotic-resistant bacteria.