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 data are updated weekly. SARS-COV-2 (the virus that causes COVID-19) 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.

SARS-COV-2 (COVID-19) Wastewater Data

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

About the Wastewater Viral Activity Level: 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. Most simply, the value associated with the Wastewater Viral Activity Level is the number of standard deviations above the baseline, transformed to the linear scale. The current Wastewater Viral Activity Level for each state and territory is categorized into minimal, low, moderate, high, or very high as follows: a Wastewater Viral Activity Level less than 1.5 is categorized as minimal, greater than 1.5 and up to 3 is low, greater than 3 and up to 4.5 is moderate, greater than 4.5 and up to 8 is high, and greater than 8 is very high.

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, it might indicate that there is a higher risk of infection. See how to protect yourself from respiratory viruses like COVID-19.

Calculating the Wastewater Viral Activity Level:

1. Data Normalization:
   • The type of data normalization used is based on the data that is 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.
2. Baseline Calculation:
   • 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.
     • 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 1^st or July 1^st, 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.
3. Wastewater Viral Activity Level Calculation:
   • 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.
4. Aggregation for National, Regional, and State Levels:
   • 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.

Data Inclusion Criteria: New NWSS 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.

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: 1) no data were reported this week from sites within the state or territory or 2) data were reported this week, but sites within the state or territory do not have a sufficient quantity of data, at least 6 weeks, to estimate the Wastewater Viral Activity Level in the last week.

Data not shown:

• 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 may not be included if there are known data quality issues.

Variants: The proportion of SARS-CoV-2 variants in each sequenced sample were assigned using Freyja (https://github.com/andersen-lab/Freyja) and averaged nationally. BA.1, BA.3 and their sublineages (except BA.1.1 and its sublineages) are aggregated with B.1.1.529. Except BA.2.12.1, BA.2.75, XBB and their sublineages, BA.2 sublineages are aggregated with BA.2. Except BA.2.75.2, CH.1.1 and BN.1, BA.2.75 sublineages are aggregated with BA.2.75. Except BA.4.6, sublineages of BA.4 are aggregated to BA.4. Except BF.7, BF.11, BA.5.2.6, BQ.1 and BQ.1.1, sublineages of BA.5 are aggregated to BA.5. Except the lineages shown and their sublineages, sublineages of XBB are aggregated to XBB. Except XBB.1.5.1,XBB.1.5.10,FD.2,EU.1.1, XBB.1.5.68 and XBB.1.5.59 sublineages of XBB.1.5 are aggregated to XBB.1.5. Except XBB.1.16.1, XBB.1.16.6 sublineages of XBB.1.16 are aggregated to XBB.1.16. Except FE.1.1, sublineages of XBB.1.18.1 are aggregated to XBB. For all the other lineages listed, their sublineages are aggregated to the listed parental lineages respectively. Any variants making up less than 5% of the national weekly average were categorized into the “Other” category.

Data Limitations: Data collected in NWSS 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 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.