Sewage surveillance as an early warning system for SARS-CoV-2 detection

In recent research, we looked at how local sewage surveillance can provide information about the prevalence of the virus by linking underground data (sewage) to above-ground data (people who tested positive and the number of people who became ill). We assessed which data provide the most reliable information on 1) the extent of the pandemic, 2) the circulation of the coronavirus, and 3) the new coronavirus variants in the city.

Patients: testing the stool

We need to assess how many virus particles an infected person excretes through their faeces to relate the amount of virus in sewage to the number of people infected with the coronavirus.

A panel of 140 people showed that 55% of the people were positive for the coronavirus in the faeces, sometimes more than two weeks after becoming ill. Of the patients who participated in the study, 45% had diarrhoea or nausea. Of this subgroup, 52% had the coronavirus in their stool. In December 2020, the first reports of the British variant (B1.1.7) arrived. By now, a large proportion of infections in the Netherlands are caused by this variant.

Sewage surveillance at the district level

Sewage surveillance is a fast and straightforward way of monitoring the circulation of SARS-CoV-2 in an entire city, district or neighbourhood. We mainly investigated how sewage water surveillance adds value to the other monitoring methods (such as GGD test strips).  In the Rotterdam-Rijnmond region, we selected several neighbourhoods with a good overlap between the sewerage pumping stations and GP practices to validate the under- and above the ground data.

Sampling cabinets have been installed at the sewerage pumping stations and have been operational since the summer of 2020. At the WWTPs, we used existing sampling equipment. We corrected the measured concentrations for the influx of rainwater, industrial water and other water that does not originate from households and dilutes the household wastewater (and thus the SARS-CoV-2 virus content).

Thanks to this normalisation step, days on which the sample is not representative for the population, for instance, because less sewage was discharged due to a pumping station failure, are filtered out. This method ensures the high quality of the measurement data.

The sewer as an objective virus indicator

Not everyone who is carrying the virus goes to the GGD testing station or the GP, but everyone does go to the toilet. At the end of 2020, the GGD had indications that relatively few people from Charlois came to the testing stations.

The high correlation between sewerage data and GGD data makes it possible to build a reasonably reliable model which can calculate the number of positive tests based on sewerage data, regardless of whether people get tested or not. These data show once more the predictive power of sewage measurements.

Comparing genetic characteristics of the viruses in sewage with those of COVID-19 patients

To investigate whether the viruses in the sewage are comparable to those of COVID-19 patients in Rotterdam, we compared the genetic characteristics of coronaviruses in 120 sewage samples with 1,300 positive patients. In addition, we investigated whether the new corona variants are present in the sewage.

We found a similar diversity of viruses in the sewage as in the patients. Hence the sewage gives a good representation of the prevalence and distribution can provide an early indication of new virus variants.

In this project, KWR is working together with Erasmus MC, GGD, general practitioners, RIVM, Waterboards, Stichting Toegepast Onderzoek Waterbeheer (STOWA), Partners4UrbanWater and Royal Haskoning DHV (RHDHV).