The foundations for the National SARS-CoV-2 Wastewater Monitoring by the BMASGPK were developed as part of the research project “Coron-A – Detection and Monitoring of SARS-CoV-2 Infections in Austria’s Population through Wastewater Analysis” (May 2020 – June 2021). Coron-A was largely funded by the BMLRT and the BMBWF, as well as by eight federal states and the Austrian Association of Cities. The project partners included AGES, the Medical University of Innsbruck, the Technical University of Vienna, the Environment Agency Austria, and the University of Innsbruck (https://www.coron-a.at).

During this research project, the following advantages of wastewater monitoring were identified:

Detection and Monitoring of viral Infections in Austria's Population through Wastewater Analysis

Pandemic surveillance in Austria currently relies primarily on individual diagnostics. Widespread testing of the population enables the fulfillment of official duties (isolation and contact tracing) as well as systematic recording of the pandemic situation and various variants (variant surveillance).

The wastewater epidemiological approach provides an additional source of information that complements individual diagnostics. The method is based on determining the total amount of viruses excreted by the population in a region through analysis of a sample taken at the inflow to the wastewater treatment plant (WWTP). Wastewater monitoring primarily serves a collective-oriented perspective. It can easily, quickly, cost-effectively, and promptly depict temporal trends and regional differences in the pandemic situation^[1]. Variant surveillance is also possible^[2]. Especially during periods of low incidence and/or reduced testing, as well as during high incidence and the resulting overload of testing capacities, assessing the situation through wastewater epidemiology is useful.

Wastewater monitoring and human testing therefore represent two complementary instruments for pandemic surveillance and control.

Due to a temporal lead, wastewater monitoring can also be used as an early warning system. An increase in viral load can indicate a potential need for action by authorities early on, allowing pandemic management measures to be justified, prepared, and initiated.

Viral load in wastewater typically begins to decline a few days earlier than the incidence derived from individual diagnostics. Therefore, wastewater epidemiology can also make a valuable contribution to evaluating the effectiveness of measures.

The broader the regional distribution of wastewater treatment plants selected, the better regional differences can be resolved and regionally limited measures justified.

Requirements for a functioning wastewater monitoring program

• Constructive collaboration between the wastewater sector, analytical laboratories, and stakeholders/authorities
• Representative distribution of sampled wastewater treatment plants
• Regular and continuous sampling, measurements, and data reporting; nationally and internationally comparable measurement and analysis methods

Fundamentals of SARS-CoV-2 Wastewater Monitoring

• Sampling: Regarding the human population, this is a full survey and not a sample survey, as all individuals in the catchment area of the WWTP are recorded. Wastewater epidemiology allows – regardless of individual willingness to participate in clinical testing strategies – continuous assessment of the extent of infection in this area.
• Sample: Since only part of the wastewater can be examined, it is a random sample in terms of wastewater volume. A 24-hour composite sample proportional to volume is considered representative. However, this involves sample variability, which can be reduced by measurements with higher temporal density.
• Regional assignment and delimitation: Wastewater-based monitoring allows regional assignments. This generally applies to the catchment area of the respective wastewater treatment plant. Additionally, upstream sampling in sewer sub-sections is possible. Suspicion-based ad-hoc investigations are feasible but require logistical flexibility.
• Time factor: Wastewater monitoring can provide very timely information about the local distribution and temporal development of SARS-CoV-2 infections. Data from wastewater monitoring can lead diagnostic tests by up to 7 days². Experience shows that wastewater sample analysis can be completed within 12 to 48 hours. The time between the end of sampling and the availability of an interpretable analysis result ultimately depends on sample logistics, analysis duration, and data logistics.
• Detection sensitivity: Every resident contributes to the total wastewater volume. However, the viral load comes only from a portion of the population. Infiltration water can cause further dilution. Results from the CoronA project¹,² indicate that with currently established methods, approximately one virus-shedding person in 10,000 can be detected.
• Costs: The cost of analyzing a composite sample for a catchment area of e.g. 100,000 residents is minimal compared to the cost of evaluating 100,000 individual tests.
• Population coverage: With a manageable number of monitored regions, relatively high population coverage can be achieved. As part of the National SARS-CoV-2 Wastewater Monitoring Program, sampling at 24 WWTPs regularly monitors about half of Austria’s population.

Benefits and Strengths of SARS-CoV-2 Wastewater Monitoring

• Wastewater-based COVID indicator: The wastewater measurement result provides not only qualitative evidence of the presence of viral fragments in the analyzed wastewater (warning/clearance) but also quantitative information from which indicators can be derived that relate to the number of virus shedders in the monitored catchment area. This allows for the derivation of “case numbers” from the measurement values, which can be used to assess the situation, including identifying temporal and spatial trends.
• Trend forecasting: Trend forecasts can estimate whether a wastewater-based indicator will remain stable, increase, or decrease over the next few days. The prerequisite for such short-term forecasts is a high temporal measurement density. Forecasts primarily apply to the catchment area of the specific WWTP. If there are multiple measurement points in a larger region (district, federal state, national territory), individual trends can be aggregated into overall trend forecasts.
• Pandemic management: The epidemiological data generated through wastewater monitoring enable early planning, communication, justification, and implementation of various pandemic management measures. With regionally comprehensive monitoring, region-specific measures can be derived based on suspicion, such as local testing initiatives, restrictions, or relaxations. Monitoring data can also be used to observe the effects of interventions and goal achievement.
• Mutations/Variants: The collected samples can be used to identify viral mutations/variants, including the derivation of temporal and spatial trends.

Calculation of trend indicators

For the weekly assessment of the viral load of a specific pathogen in the wastewater of a sewage treatment plant, a trend indicator is calculated that describes the development of the concentration over time. This is based on the measured values of the viral load in the last four calendar weeks.

To determine the trend, a linear regression analysis is performed using time (in weeks) as the independent variable and viral load as the dependent variable. The slope of the resulting regression line describes the average weekly change in viral load. To put this change into a relative context, the slope is divided by the mean viral load of the four weeks under consideration. The result is a percentage change per week.

This value is then classified into one of four trend categories:

• A decrease is present if the change value is below –10% per week.
• A stable trend is assumed for values between –10% and +10%.
• A moderate increase corresponds to values between +10% and +50%.
• A sharp increase is reported for values above +50% per week.