Wastewater Surveillance: A National Concept for Germany-A Refined Approach to Surveillance Site Selection.

During the coronavirus disease-2019 (COVID-19) pandemic, wastewater-based surveillance (WBS) gained renewed importance by enabling nationwide assessment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) circulation independent of individual testing. In Germany, WBS was established by a series of initiatives, including the AMELAG project, established in 2022. As the pandemic phase waned, the extensive surveillance infrastructure-comprising around 170 wastewater treatment plants (WWTPs)-was scaled down. A subset of 53 WWTPs was selected by a hierarchical set of criteria to ensure continuation of WBS in 2025, resulting in a reduced population coverage (26% instead of 37%), preserving the monitoring of infection dynamics while improving operational efficiency. The multi-stage selection approach integrated operational experience and performance data of WWTPs collected between November 2022 and July 2024, including wastewater characteristics and laboratory quantification success, minimum population coverage across all administrative regional states of the country, and statistical quality metrics such as the frequency of outliers and implausible inflexion points or the deviation from LOESS regression trends. Additional consideration was given to sites of extended research relevance. Reducing the number of WWTPs by more than two-thirds did not result in notable deviations in the aggregated national SARS-CoV-2 viral load profile. However, the evaluation was limited to SARS-CoV-2 data, despite ongoing expansion of the WBS network to include additional pathogens. Overall, the data-driven site-selection framework, developed from scientific and operational criteria, ensures the sustainable continuation of the nationwide WBS system.