About Wastewater Data

Wastewater monitoring data are most useful when used with other data, such as hospital visits or clinical testing data. Wastewater data are primarily used in four ways:

• Monitoring for the presence of infections in a community, regardless of whether the infections cause severe illness, mild illness, or no symptoms at all. By acting as an early warning system, wastewater monitoring can detect small changes early and take quick action to prevent further infections.
• Tracking trends in infection in a community whose sewage flows into a wastewater treatment plant (known as a sewershed).
• Tracking infection trends within a state, region, and nationally
• Monitoring variants of a virus that are causing infections in a community

More data over time can give health departments with better, more reliable insights into trends that are happening in a community, state, region, and the nation. Public health officials watch for sustained increasing levels of viruses and bacteria in wastewater and use these data to inform public health decisions. State and local health officials track a variety of data and put this information together to understand the local situation and decide how to best respond.

CDC’s National Wastewater Surveillance System (NWSS) works with partners to ensure that wastewater data are collected, analyzed, and interpreted using scientifically sound sampling strategy, testing strategy, and valid data processing, analysis, and interpretation techniques.

Wastewater monitoring can detect viruses spreading from one person to another within a community earlier than clinical testing and before they go to their doctor or hospital. It can also detect infections without symptoms. If you see increased Wastewater Viral Activity Levels of SARS-CoV-2 or the relative influenza A virus wastewater level percentile, it might indicate that there is a higher risk of infection. See how to protect yourself from respiratory viruses like COVID-19 and Flu.

Wastewater data are updated weekly. SARS-COV-2 (the virus that causes COVID-19) and influenza A data are updated on Fridays, and mpox data are updated on Wednesdays. Analyzed data are available for download from data.cdc.gov. The full NWSS dataset, including raw data, is available by submitting a data request to NWSS@cdc.gov.

NWSS recommends wastewater testing twice per week at each site across the United States and in territories and select tribal nations. For the latest number of sites reporting data, see About CDC’s Wastewater (NWSS) Program.

The following are more in-depth methods for how wastewater data are analyzed and displayed for:

• Wastewater Viral Activity Level for SARS-COV-2 (COVID-19), influenza A, and RSV
• Avian Influenza (H5) Detections
• SARS-COV-2 Variants

Note:  Wastewater surveillance is an evolving science, and CDC may update methods and visualizations to improve the understandability of wastewater data.

Wastewater Viral Activity Level: SARS-COV-2, Influenza A, RSV

The Wastewater Viral Activity Level is a calculated measure that allows us to aggregate wastewater sample data to get state/territorial, regional, and national levels and see trends over time.

The value associated with the Wastewater Viral Activity Level is the number of standard deviations above the baseline, transformed to the linear scale. The formula is Wastewater Viral Activity Level = e ^{# of standard deviations relative to baseline}.

The current Wastewater Viral Activity Level for each state and territory is categorized into minimal, low, moderate, high, or very high as follows:

Calculating the Wastewater Viral Activity Level

• Data are normalized based on the data that are submitted by the site.
  • If both flow-population and microbial normalization values are available, flow-population normalization is used.
• After normalization, all concentration data is log transformed.

• For each combination of site, data submitter, PCR target, lab methods, and normalization method, a baseline is established. The “baseline” is the 10th percentile of the log-transformed and normalized concentration data within a specific time frame. Details on the baseline calculation by pathogen are below:
  • SARS-CoV-2
    • For site and method combinations (as listed above) with over six months of data, baselines are re-calculated every six calendar months (January 1st and July 1st) using the past 12 months of data.
    • For sites and method combinations with less than six months of data, baselines are computed weekly until reaching six months, after which they remain unchanged until the next January 1st or July 1st, at which time baselines are re-calculated.
  • Influenza A and RSV
    • For site and method combinations (as listed above) with over twelve months of data, baselines are re-calculated every August 1st using all available data in the previous 18 months.
    • For sites and method combinations with less than twelve months of data, baselines are computed weekly until reaching twelve months, after which they remain unchanged until the next August 1st, at which time baselines are re-calculated.
• The standard deviation for each site and method combination is calculated using the same time frame as the baseline.

• The number of standard deviations that each log-transformed concentration value deviates from the baseline (positive if above, negative if below) is calculated.
• This value (x) is then converted back to a linear scale (by calculating e^x) to form the Wastewater Viral Activity Level for the site and method combination.
• The Wastewater Viral Activity Levels from a site are averaged by week for all figures.

• We calculate the median Wastewater Viral Activity Levels among sites at national, regional, and state levels, excluding data from site/method combinations with less than 6 weeks of data for SARS-CoV-2, and 10 weeks of data for Influenza A and RSV.

Data Inclusion Criteria – SARS-COV-2, Influenza A, RSV Wastewater Viral Activity Level
SARS-CoV-2: New wastewater sampling sites, or sites with a substantial change in laboratory methods are included in national, regional, state, or territorial median values once there are at least 6 weeks of samples reported for that location.

Influenza A and RSV: New wastewater sampling sites, or sites with a substantial change in laboratory methods, are included in national and state or territorial median values beginning on August 1^st of each year once there are at least 10 weeks of samples reported for that pathogen. Data must be reported by October 1^st of that year to be included in national and state or territorial median values for that respiratory virus season. If data are reported after October 1^st of that year, they will not be displayed until August 1^st of the following year.

Insufficient SARS-CoV-2, Influenza A, and RSV Data: States or territories without sufficient data to estimate the wastewater viral activity level for the previous week are indicated as “Insufficient Data.” A label of “Insufficient Data” means that either:

• No data were reported this week from sites within the state or territory
• Data were reported this week, but sites within the state or territory do not have enough data to estimate the Wastewater Viral Activity Level in the last week. This is at least 6 weeks of data for SARS-CoV-2, at least 10 weeks of data for Influenza A and RSV with consistent methodology.

State/Territory Pages Not Displayed: If states do not have historical data available, the state/territory page is not available.

• The following states/territories do not have historical data for Flu A: Montana, North Dakota, Rhode Island, Guam + other territories
• The following states/territories do not have historical data for RSV: Montana, North Dakota, Guam + other territories

Data with High Variability: States or territory data may have high variability in data displayed. These data may not be representative of their entire state/territories. This could be for a few reasons, such as:

• Data could represent a small proportion of the population (e.g. <5%).
• A small number of sites are reporting for that state/territory.

Data that have these characteristics or other data anomalies are indicated on data visualizations.

Data Approximation for Site-Specific Locations: Locations for wastewater treatment plants are approximated. Locations may be also adjusted to allow sites within close proximity to be more easily distinguished on a map.

Avian Influenza A(H5) Wastewater Data

A wastewater academic partner developed and validated an assay to detect the avian influenza A(H5) hemagglutinin gene in wastewater. This assay was initially implemented in May 2024 for routine testing in 193 sites, which was reduced to 152 sites on July 1, 2024, across 41 states. State and local partners may be using other assays to test and report data. In July 2024, CDC’s wastewater data pipeline was updated to receive avian influenza A(H5) subtyping results.

About the influenza A(H5) virus detection metric used in wastewater: The avian influenza A(H5) virus detection metric used in wastewater is calculated and displayed for individual sampling sites and reflects whether virus was detected within samples collected over the previous week. An individual sampling site is typically a wastewater treatment plant but may be a location in the sewershed before wastewater enters the wastewater treatment plant.

Calculating the avian influenza A(H5) virus detection metric in wastewater:

1. Avian Influenza A(H5) Virus Metric Calculation:

• • When multiple samples are collected, or multiple laboratories test samples from the same site in a given week and avian influenza A(H5) virus is detected in any sample collected during that week, the site is classified as a detection.
  • Results reporting avian influenza A(H5) virus at concentrations above the limit of detection are displayed as detections on CDC’s wastewater dashboard. Limits of detection are calculated by the laboratory doing the analysis and are specific to the assay and laboratory methodology.

Data Inclusion Criteria
States or territories that do not test for avian influenza A(H5) virus or have not reported data in the past two months will not have sites displayed on the map. Sites that have not submitted avian influenza A(H5) data in the previous week are labeled as ‘No samples in last week.’

SARS-COV-2 Variants

The proportion of SARS-CoV-2 variants in each sequenced sample were assigned using Freyja and averaged nationally. All lineages not enumerated in COVID-19 Variants in Wastewater dashboard are aggregated with their parent lineages, based on the Pango statement of nomenclature rules. Any variants making up less than 5% of the national weekly average were categorized into the “Other” category.

CDC does not publicly display data that meet any of the following criteria, including related to privacy concerns:

• Data from sewersheds that serve fewer than 3,000 people
• Data from facility or institution-specific sampling locations
• Data from tribal communities
• Data from sewersheds with known data quality issues.

Data collected in CDC’s National Wastewater Surveillance System may over-represent certain populations within a state or territory. For example, most jurisdictions report data from municipal sewer system samples and may have limited inclusion of populations that rely on other non-sewered sanitation systems (e.g. septic tanks). They may also over-represent urban and suburban populations because of wastewater utility locations and the associated sewershed. As a result, these data may not accurately depict the full picture of viral activity for the entire state or territory.

The data presented are preliminary and may change as more data are received. Differences in the data presented by CDC and state health departments likely represent differing levels of data reporting; data presented by the state are likely the more complete ones. Additionally, there may be differences in data visualization approaches that impact how data are displayed.

Specific wastewater data limitations for each pathogen will be available in the data footnotes or About the Data sections:

• SARS-COV-2
• Influenza A
  • Avian Influenza A (H5)
• RSV
• Mpox