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Short answer
Wastewater surveillance is a population-level monitoring method that measures pathogen genetic material in sewage. In the United States, CDC’s National Wastewater Surveillance System (NWSS) uses it as an early signal that viruses may be spreading in a community, even before people go to a doctor or hospital.
In public health research and epidemiology, it gives an aggregate view of a whole sewershed, the area drained by one treatment plant or sampling point.
Definition and scope
Wastewater surveillance means collecting sewage samples, concentrating and extracting the genetic material in them, and quantifying specific targets such as viral RNA. Because a sample pools the waste of many people, it describes a community rather than any one person.
The National Wastewater Surveillance System (NWSS) is CDC’s national program. Current cdc wastewater surveillance pages list the pathogens it reports on as influenza A, COVID-19, RSV, measles, avian influenza A (H5) and mpox. The nwss program receives data from more than 1,000 sites each week, and CDC recommends that each site test twice per week.
How it works in practice
Programs generally follow a four-stage workflow: sample collection, concentration and extraction, molecular quantification, and normalization. The technical outline below describes common laboratory practice; the CDC pages cited in Sources point to the pathogen-specific “About the Data” sections for exact NWSS calculations.
Sample collection
Samples are usually taken at the treatment plant inflow. Composite samples, collected over a period such as 24 hours, smooth out the daily swings in sewage flow that a single grab sample can miss.
Concentration and extraction
Sewage holds low concentrations of viral genetic material in a complex matrix, so laboratories concentrate the sample and then purify the nucleic acid before testing.
Detection and quantification
- RT-qPCR: reverse transcription quantitative PCR estimates target copies against a calibration curve.
- RT-ddPCR: droplet digital PCR partitions each reaction into many droplets and counts positive ones, giving absolute counts without a standard curve.
- Sequencing: CDC also analyzes wastewater sequencing data deposited in NCBI, with human reads removed before use.
Normalization
Raw concentrations vary with rainfall, industrial discharge and population changes, so laboratories commonly adjust for wastewater flow or for a human fecal marker such as pepper mild mottle virus (PMMoV). These adjustments are meant to make results comparable across sites and days.
CDC makes the data available on data.cdc.gov, where it can be downloaded or accessed by API, and updates it every Friday. CDC runs quality checks before publishing, and it does not publicly show data from sewersheds serving fewer than 3,000 people. Teams that use such data to assess community programs may also find the CDC program evaluation framework useful.
Related concepts
- Community sewershed monitoring: the NWSS model, sampling at treatment plants.
- Sequencing-based monitoring: analysis of genetic sequences from wastewater to follow variants.
For how neighboring fields differ, see the comparison public health vs environmental health.
Where it matters for research
CDC states that wastewater monitoring can signal viral spread before people seek care, and that this early signal can be used to prepare healthcare providers and hospital systems for upcoming increases in visits and hospitalizations. Researchers who cite wastewater data in grant applications may connect it to NIH project narrative public health relevance statements, and observational studies that use it can be reported with the STROBE checklist.
Common misunderstandings
- Wastewater data show exact case counts. CDC states it is challenging to use wastewater data to estimate how many people in a community are sick, because that depends on how much virus is shed, which varies with severity and viral strain.
- Everyone is covered. CDC notes that communities relying on septic tanks or other non-sewered sanitation, or in rural areas, might not participate in wastewater monitoring.
- Results are instant. CDC gives about 5 to 7 days from toilet flush to results.
FAQ
What is the National Wastewater Surveillance System (NWSS)?
NWSS is CDC’s national program for wastewater monitoring. It reports on pathogens including influenza A, COVID-19, RSV, measles, avian influenza A (H5) and mpox, using data from more than 1,000 sites weekly.
How long does it take to get wastewater results?
CDC states that going from toilet flush to results takes about 5 to 7 days.
Can researchers access CDC wastewater data?
Yes. CDC states that wastewater data can be downloaded or accessed by API on data.cdc.gov (search “wastewater”). CDC also hosts a GitHub repository, github.com/CDCgov/NWSS, with shared tools and supporting resources.
Can wastewater data tell how many people are sick?
Not reliably. CDC says it is challenging to estimate the number of sick people from wastewater data alone.
Sources
- Centers for Disease Control and Prevention. Wastewater Surveillance (NWSS overview and data). https://www.cdc.gov/nwss/about.html (checked 2026-11-07).
- Centers for Disease Control and Prevention. NWSS data methods and sharing. https://www.cdc.gov/nwss/data-methods.html (checked 2026-11-07).
- Centers for Disease Control and Prevention. About wastewater data. https://www.cdc.gov/nwss/about-data.html (checked 2026-11-07).
- National Academies of Sciences, Engineering, and Medicine. Wastewater-Based Disease Surveillance for Public Health Action. https://nap.nationalacademies.org/catalog/26809/wastewater-based-disease-surveillance-for-public-health-action (checked 2026-11-07).
