Appropriate public health interpretation of wastewater surveillance data depends on understanding the surveillance sampling strategy and testing limitations, as well as valid data processing and analysis. Wastewater surveillance data are primarily used in three ways:
- Monitoring for presence of infection within a community.
- Tracking trends in infection within a community.
- Screening for infections at a targeted site (e.g., building or facility) to trigger additional individual-based testing and mitigation measures. Review CDC’s guidance on targeted wastewater surveillance to use wastewater surveillance data for screening.
Wastewater surveillance data collected at the municipal level, when analyzed appropriately, can provide information on:
- Presence of infected individuals contributing to a wastewater system.
- Infection trends within the community contributing to a wastewater treatment plant (known as a “sewershed”). Sewersheds with largely transient populations, such as areas with high tourism, may provide less stable signals which should be considered when designing the wastewater surveillance plan for public health action.
Detection of SARS-CoV-2 in wastewater depends on the sampling design, the sensitivity of the test used, and the amount of SARS-CoV-2 being shed by the community sampled. If SARS-CoV-2 is detected in wastewater, this means that there is at least one individual in the sewershed shedding SARS-CoV-2. Whether individual(s) are infectious or symptomatic cannot be determined from wastewater surveillance data. If SARS-CoV-2 is not detected in wastewater, this could indicate that there is no SARS-CoV-2 in the sampled community or that the concentration of virus in wastewater is below the level the test can detect.
Low virus levels in the wastewater of a community could indicate: 1) a small total number of persons are shedding the virus into the sewer system; or 2) the amount of virus being shed per infected or recently infected person is low. The minimum number of individuals shedding SARS-CoV-2 into the system needed to detect a viral RNA signal in wastewater is not known at this time. As more information on fecal shedding titers are collected, it may be possible to determine the number of individuals shedding the virus within a sewershed needed to detect SARS-CoV-2 RNA in wastewater.
Wastewater trend classification is the statistical analysis of changes in the normalized concentration of SARS-CoV-2 in wastewater (i.e. not by qualitative visual assessment). Trends in these wastewater data can be used to assess COVID-19 trends (reported and unreported) within the community contributing to the sewer system. Trends can by classified into categories based on the duration and direction of change in viral quantities. The frequency of wastewater sampling will dictate the time period for which trends can be assigned.
A benefit of trend analysis is that:
- Data from wastewater treatment plants can be compared, despite differences in population size and wastewater volume.
- Trends in wastewater may be known prior to COVID-19 reported case trends, given that normalized concentration of SARS-CoV-2 in wastewater have been shown to coincide or lead with new reported cases within a sewershed by days.