Wastewater surveillance (WWS) has become a prominent method for tracking community health trends, especially for infectious diseases like COVID-19. This non-invasive approach involves monitoring sewage for genetic material, providing a pooled, population-level sample of viral shedding. WWS offers an alternative and complementary data source to traditional individual clinical testing, assessing the overall burden of the virus in a community. Understanding the utility of this method requires examining the science behind the detection process, how its “accuracy” is defined for a population, and the environmental factors that influence its results.
The Science of Detection: How Wastewater Surveillance Works
Wastewater surveillance relies on the biological fact that individuals infected with SARS-CoV-2 shed viral genetic material in their stool, often even before symptoms appear or if they remain asymptomatic. This viral ribonucleic acid (RNA) travels through the sewage system, effectively creating a composite sample of the entire population served by that sewer network, known as a sewershed. Samples are typically collected at wastewater treatment plants, or sometimes at specific manholes to monitor smaller, localized areas.
The collected wastewater is transported to a laboratory, where a concentration and extraction process isolates the viral RNA fragments from the complex biological and chemical components of the sewage. Scientists then use a highly sensitive technique called quantitative Polymerase Chain Reaction (qPCR) to detect and measure the concentration of the SARS-CoV-2 RNA. The final result is a measurement of the viral load, typically expressed as gene copies per liter of wastewater, which serves as a proxy for the total amount of virus being shed.
Interpreting “Accuracy” in Community Health Data
The concept of accuracy in wastewater surveillance differs significantly from that of an individual diagnostic test. WWS does not aim to diagnose a single person, but rather to assess the overall prevalence and trajectory of the virus within a defined population. Its primary strength lies in its ability to be unbiased by factors like access to healthcare, testing availability, or an individual’s decision to seek a clinical test. This makes the data a consistent measure of community viral activity, especially as reported case counts become less reliable due to at-home testing.
Wastewater data functions as a leading indicator, meaning that increases in the concentration of viral RNA often precede a rise in reported clinical cases or hospitalizations by several days up to two weeks. This is because viral shedding in stool can begin before the onset of respiratory symptoms that prompt a person to get tested. Studies have shown strong correlations between the trend of wastewater viral concentrations and subsequent hospitalization rates, confirming its utility as a reliable forecasting tool.
While WWS is a strong predictor of trends, it is not used to precisely calculate the exact number of infected people in the community. The relationship between the concentration of viral RNA in sewage and the number of active infections is complex and influenced by many variables. However, wastewater concentrations are a significantly better predictor of true community prevalence than reported case counts alone, which are limited by underreporting and testing biases.
Environmental and Technical Factors Influencing Results
The viral RNA concentration measured in a wastewater sample is not a perfect, direct reflection of the number of infected individuals because various environmental and technical factors introduce variability. One significant factor is dilution, caused by high volumes of non-fecal water entering the sewer system, such as from heavy rainfall or snowmelt. Increased water flow effectively lowers the concentration of viral RNA in the sample, potentially masking a true increase in community infection.
The stability of the viral RNA is another concern, as it can degrade over time and with exposure to certain conditions within the sewer system. Factors like high wastewater temperature, pH levels, and the travel time of the sewage can all affect the integrity of the RNA fragments. Furthermore, technical differences in laboratory procedures, such as the specific method used for RNA extraction or the volume of the sample analyzed, can lead to variations in the final reported concentration.
To account for these fluctuations, laboratories often normalize the SARS-CoV-2 RNA concentration by measuring a stable human fecal marker, such as the Pepper mild mottle virus (PMMoV) or specific bacterial DNA. This normalization helps adjust the data for changes in flow or dilution, providing a more accurate picture of the viral load relative to the amount of human waste in the sample. The size of the sewershed also plays a role, as larger populations generally lead to greater dilution and increased complexity in data interpretation.
Practical Application: Leveraging Wastewater Trends
Despite the inherent variability, the practical value of wastewater surveillance lies in its ability to capture infection trends in a way that is not possible through individual testing. Public health departments use the relative changes in viral load to create an early warning system for COVID-19 surges. When a steady increase in RNA concentration is observed, it suggests that community transmission is rising, allowing officials to prepare hospital systems and communicate risk to the public before a spike in clinical cases occurs.
Wastewater samples also allow for the genomic sequencing of the SARS-CoV-2 RNA, which is an invaluable tool for tracking the emergence and circulation of new variants. Because the sample pools genetic material from thousands of people, it can often detect a new variant days or weeks before it is identified through clinical sequencing. This information guides decisions on vaccine and testing strategies, particularly when clinical testing rates are low or inconsistent. The data is leveraged to make targeted public health decisions, such as deploying mobile testing sites or focusing vaccination campaigns on high-risk areas.