Sewage is not only to be regarded as unwanted waste but can also provide useful information. This fact has attracted some media interest during the COVID-19 pandemic. Since SARS-CoV-2, like many other viruses, is excreted in the feces, it seems plausible that virus particles also appear in wastewater and can be detected therein. But is wastewater surveillance really a suitable epidemiological measure?
When searching for articles on wastewater monitoring in scientific literature databases such as PubMed, it quickly becomes obvious that the number of hits has increased significantly in recent years. Understandably, articles dealing with SARS-CoV-2 have been dominating since 2020. A recent systematic review by a Singapore research team identified a total of 763 studies addressing the correlation of viruses in wastewater and COVID-19 cases in the population and ultimately included 92 studies in the analysis [1]. All but four reported a possible link between wastewater signals and community COVID-19 cases. However, it also became apparent that wastewater signals preceded confirmed cases by up to 63 days and were therefore not suitable for estimating the number of people currently infected. The review also draws attention to the fact that positive wastewater samples were not uniformly defined in the studies, and methods were mostly not validated. Thus, the research team concludes that wastewater monitoring has some advantages over traditional epidemiological methods (e.g. low time consumption, low cost, no exposure risk) but can only complement clinical surveillance (e.g. through PCR testing). If uniform standards are established and statistically reliable data for the allocation of case numbers are generated, the method could certainly allow countries to react more actively to pandemic dynamics in the future [1].
During the second pandemic wave, certain districts in Germany had very high incidences at times. A current case study from the German district Berchtesgadener Land describes how comprehensive SARS-CoV-2 biomarker wastewater monitoring for the early detection of changes in local infection dynamics and the occurrence of virus variants supplemented the measures taken by the crisis management team from November 2020 on [2]. Samples were taken twice a week in an automated manner from nine municipal sewage plants and three sampling points from the sewage system and represented approximately 100,000 inhabitants (approx. 95%) of the district. Thanks to the rapid forwarding, results were available after a maximum of 48 hours and were fed into a regional dashboard in which, among other things, new infections of the last 7 days as well as per day were documented. It was found that the measured abundances reflected the changes in infection dynamics very well and about 10 days earlier than official case numbers. The case study thus demonstrated that innovative crisis management is possible and that wastewater surveillance helps predict both increasing and decreasing infection rates [2].
Even though Shah et al., based on the international scientific evidence, currently still see methodological weaknesses in wastewater surveillance and rather consider it complementary to common epidemiological methods [1], the European Commission has been recommending comprehensive wastewater monitoring to the member states since 2021. The urban wastewater treatment directive is currently under review, allowing the submission of a legislative proposal in 2022 [3]. As sewage surveillance has been playing a major role in the Global Polio Eradication Initiative for several years [4], a stronger implementation of wastewater monitoring would not be entirely new. It also enabled the recent detection of vaccine-derived poliovirus in London sewage, which is not an immediate health concern but indicates the fight against the disease is not over.
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