What to know
- CDC uses various methodologies to ensure that wastewater monitoring data are accurate, comparable, and easy to understand.
- CDC hosts a GitHub data repository to support collaboration and code sharing.
- The wastewater viral activity level (WVAL) was developed to compare the current levels of common respiratory viruses to low viral levels at that location over the last 24 months.
Ensuring high-quality data
CDC receives data from about 1,500 wastewater monitoring sites across the United States each week. CDC's wastewater monitoring program recommends that each site conducts wastewater testing twice per week. CDC's experts work hard to ensure these data are of high quality, comparable across sites, and understandable.
Protecting data quality
Before publishing updated wastewater data, CDC completes data quality checks and reviews the data to ensure there aren't any issues.
Ensuring data comparability
CDC developed the wastewater viral activity level (WVAL) metric to allow data to be compared between wastewater monitoring sites and combined into state/territorial, regional, and national levels.
The WVAL also standardizes data to see trends over time.
Increasing understanding of data
CDC works with state, tribal, local, and territorial health departments to ensure that metrics and visualizations are easy to understand and use for public health and individual decision making.
Continuous improvement
Wastewater monitoring is an evolving science, and CDC may update methods and visualizations to improve how wastewater data are shared.
GitHub data repository
CDC's wastewater program hosts a GitHub data repository to support collaboration and to share code among jurisdictions, CDC, and the public to advance wastewater monitoring activities. The repository hosts jurisdiction-contributed projects, shared tools, and supporting resources intended to promote transparency, reproducibility, and innovation in public health monitoring. All content is publicly available and maintained in accordance with CDC open-source and privacy standards.
WVAL methodology updates
On August 15, 2025, CDC updated the methodology used to calculate the WVAL. This update was made as a result of a revalidation process to ensure continuous quality improvement. These updates were applied to all historical data to ensure data can be compared over time.
- Normalized data adjusts for factors such as flow rate or population size to help compare across time or locations.
- CDC decided to switch to calculating the WVAL using non-normalized data to help ensure that increases or decreases reflect changes in the amount of virus in the wastewater, not the factors used to normalize the data (such as flow rate). This also simplifies WVAL calculations and lowers the risk of data processing errors.
- The WVAL metric itself is a method of standardizing site-level data; therefore, no additional normalization is needed.
Previously, the baselines were 12 months for COVID-19 and 18 months for influenza A and RSV. The longer look-back period includes more data in the baseline measurements, making WVALs more stable and less prone to extreme values. When we see fluctuations, we are more confident this reflects increases or decreases in viral activity in a community.
A minimum of 8 weeks of data are needed to help avoid extreme fluctuations that may occur with less data, impacting our ability to accurately interpret the the data. Previously, 6 weeks were required for COVID-19 and 10 weeks were required for influenza A and RSV.
This updated timing better aligns with seasonal fluctuations in cases.
This provides additional context for comparison.
- CDC reviews the value ranges each year ahead of the fall respiratory virus season using multiple statistical methods at the state/territory, regional, and national levels. The review process helps ensure that the ranges align with categorizations used in clinical surveillance platforms and reflect updated baselines.
- The transition from normalized data to non-normalized data resulted in a shift to an overall smaller range of WVAL values. To account for the smaller range of values, the ranges associated with each WVAL category were adjusted accordingly and now more accurately represent viral activity within a community.
Understanding the WVAL
The WVAL compares current levels of COVID-19, influenza A, or RSV to low viral levels at that location over the last 24 months.
Why it is important
For You
If you see increased WVALs, it might mean there is a higher risk of infection. You can use these data to make informed decisions on how to protect yourself and others from respiratory viruses.
Public health officials use data from wastewater (sewage), along with clinical data, to understand virus levels in a community.The WVAL helps public health officials:
- See the bigger picture across a state, region, or the United States
- Compare data across cities, states, and time
- Spot unusual increases in COVID-19, influenza A, or RSV
What data to look at
These show the current status of the virus in your community
Consider whether the virus is increasing, decreasing, or staying the same
Look at other data, such as clinical data, to see if they're showing similar trends.
Example resource: Respiratory Illnesses Data Channel
How it is calculated
Step 1: Group data together
To help ensure that we're comparing similar data, group the amount of virus present in a sample by:
- Which virus is being measured
- Which site the sample came from
- Which lab and laboratory method are used
Step 2: Validate data
To ensure data are ready for comparison:
- Log transform the data: This helps us to see overall trends more clearly by reducing the influence of extreme values.
- Remove unusually high or low values (outliers): Unusually high or low values can skew the data, leading to inaccurate interpretation. If future data continue the high or low trend, the data are no longer removed.
Step 3: Determine low levels for each site
When looking at data from the past 2 years, select the value that is higher than 10% of values to be used as a baseline for comparison. The baseline shows what is considered a typical low level of the virus for a site. The WVAL shows how far away current levels are from low.
Step 4: Compare current levels to baselines
To calculate the WVAL, compare the current amount of virus to the baseline level, then divide that by the standard deviation.
This provides a standard number of how many "steps" (standard deviations) current levels are from low levels, allowing us to compare levels between sites. The larger the WVAL value is, the further away from low levels it is.
Step 5: Identify average and median values
For each week, average the values. The median (middle) value is used to represent each state, region, and the United States. This approach helps show weekly and big-picture trends.
Step 6: Categorize Values
Group the WVAL values into one of five categories (see below for thresholds): Very low, low, moderate, high, or very high. These categories help provide a quick understanding of the levels.
Each virus has its own thresholds because each virus behaves differently.
| Very Low | Low | Moderate | High | Very High | |
|---|---|---|---|---|---|
| Influenza A | Up to 2.7 | Greater than 2.7 and up to 6.2 | Greater than 6.2 and up to 11.2 | Greater than 11.2 and up to 17.6 | Greater than 17.6 |
| COVID-19 | Up to 2 | Greater than 2 and up to 3.4 | Greater than 3.4 and up to 5.3 | Greater than 5.3 and up to 7.8 | Greater than 7.8 |
| RSV | Up to 2.5 | Greater than 2.5 and up to 5.2 | Greater than 5.2 and up to 8 | Greater than 8 and up to 11 | Greater than 11 |
How WVALs are aggregated
The median of site-level WVALs
The median of state/territory-level WVALs
The median of regional-level WVALs
States and territories may have a higher density of sampling sites in certain geographic areas, so the median WVAL may not represent the WVAL for every community in the state or territory.
WVAL inclusion criteria
COVID-19
New wastewater sampling sites and sites with a substantial change in laboratory methods are included in national, regional, state, or territorial median values once there are at least 8 weeks of samples reported for that location.
Influenza A and RSV
New wastewater sampling sites and sites with a substantial change in laboratory methods are included in national and state or territorial median values beginning on August 1 of each year once there are at least 8 weeks of samples reported for that pathogen. Sites must begin sampling by October 1 each year for their WVAL data to be displayed on the website. If sites begin sampling after October 1, their WVAL data will not be displayed until August 1 of the following year.