Wastewater Monitoring: Respiratory Virus Levels in Wastewater

This interactive dashboard includes the following sections (see definitions and methods for more information).

Description	Section
A legend with respiratory virus concentration levels categorized by color.
A graph showing the statewide average levels over time for the selected respiratory virus.
Current statewide average concentration level for the selected respiratory virus.
Current statewide average trajectory of the selected respiratory virus (increasing, decreasing, or stable).
A map of Wisconsin displaying the most recent weekly level of the selected respiratory virus for each location across the state and the average level for each public health region.
A heat map (colored squares) showing the weekly average level of the selected respiratory virus for each location over time.
A scatter plot showing the normalized concentrations for all wastewater samples of the selected respiratory virus at the selected location over time. Colors of the points indicate the concentration level for each sample, and the line represents the trend over time.

Wastewater sample collection and testing

Participating wastewater treatment facilities in Wisconsin collect and submit samples of untreated wastewater for testing at the Wisconsin State Laboratory of Hygiene (WSLH). Facilities typically send samples once or twice a week, which are received, processed, and tested for different pathogens. For the respiratory viruses shown on this page (influenza A, influenza B, RSV and SARS-CoV-2), viral RNA is concentrated and extracted from the sample and then tested using digital polymerase chain reaction (PCR).

Normalization of respiratory virus concentrations

Raw concentrations for each respiratory virus are normalized by the population size and daily flow rate of the wastewater treatment facility. Outliers are removed, and the final normalized concentrations for each virus are analyzed and displayed on the dashboard based on the virus and location.

Categorizing wastewater levels for each location

The colored squares on the dashboard show levels in each community for the selected respiratory virus, ranging from very low to very high.

Weekly levels

The colored squares on the heat map (colored squares over time) and the map of Wisconsin show the selected respiratory virus levels by week across all participating communities, ranging from very low to very high.

For each community, levels are summarized using data from the most recent week of samples. These values reflect the average across samples collected during that week (typically one or two samples), to represent overall weekly levels.

The weekly average concentration for each location is compared to the historical levels for the selected respiratory virus observed at that location. The amount of historical data available for comparison varies by location and is only used during periods of disease activity (for example, in-season), and when laboratory methods are consistent. Weekly levels are assigned to one of five categories based on comparison with the historical levels:

• Highest 20% (very high)
• 60th –80th percentile (high)
• 40th –60th percentile (moderate)
• 20th –40th percentile (low)
• Lowest 20% (very low)

Individual samples and trends

Individual sample results (shown as colored dots on the trend graph) reflect concentration and associated levels for the selected respiratory virus measured on specific collection dates.

The samples are categorized using the same approach as weekly levels, by comparing each result to the historical range for that community. Individual samples are not averaged together before categorization. Due to high sample-to-sample variability in wastewater measurement, a trend-line is provided that shows the trajectory and the level over a rolling five-sample window, which uses a statistical smoothing technique called a spline.

For the region

The regional activity level summarizes respiratory virus wastewater levels across participating wastewater treatment plants within each public health region. At each site, respiratory virus wastewater measurements are combined and summarized as a weekly average, as described above. The weekly value for each location is then compared with that site’s historical wastewater data and assigned a category of very low, low, moderate, high, or very high.

To calculate the regional category, each site’s weekly category is converted to a numeric score:

• Very low = 1
• Low = 2
• Moderate = 3
• High = 4
• Very high = 5

The site scores are combined within each public health region using a population-weighted average. This means that wastewater treatment plants serving larger populations contribute more to the regional result than plants serving smaller populations.

The population-weighted score is then converted back into a regional activity category:

• Less than 1.5: Very low
• 1.5 to less than 2.5: Low
• 2.5 to less than 3.5: Moderate
• 3.5 to less than 4.5: High
• 4.5 or higher: Very high

A regional result is only reported when at least half of the expected sites in the region report, with a minimum of two reporting sites. If this requirement is not met, the regional activity level is not displayed.

Statewide average respiratory concentration (line graph)

The statewide average concentration for the selected respiratory virus is displayed as a line graph over time on the dashboard. The line represents the average concentration (in millions of gene copies per person per day) of the selected respiratory virus across all sites in Wisconsin over time. Individual sites may have levels and trends that are different from the statewide average, so select your local community for a detailed view.

To calculate a statewide average over time, we use the following methods:

1. For each site and respiratory virus, the concentrations are normalized by population and flow rate, and outliers are removed.
2. These normalized levels are then adjusted to account for differences in laboratory methods used at different sites over time.
3. Levels are then averaged across the past seven days for each site and averaged across all sites to produce a daily adjusted statewide average.
4. This line is then smoothed (using a statistical method called a double exponential moving average) to make changes over time easier to see.
5. The statewide average will not be displayed on the graph until there are at least 50% of sites represented in the calculation.

Statewide average respiratory virus level (category)

The most recent statewide average concentration for each respiratory virus is compared with historical statewide averages and is assigned to one of five categories:

• Highest 20% (very high)
• 60th – 80th percentile (high)
• 40th –60th percentile (moderate)
• 20th – 40th percentile (low)
• Lowest 20% (very low)

The most recent category is displayed on the dashboard as a colored box titled “current state level”.

Statewide respiratory virus trajectory

The statewide trajectory for each respiratory virus shows whether the current statewide level for the selected virus is increasing, decreasing, or stable compared to the average level calculated over the prior three days.

The level of influenza A, influenza B, RSV and SARS-CoV-2 at a given site indicates the average amount of these viruses detected in samples from that site in the past week compared to all levels measured at that site over time. A higher level indicates that more virus is circulating within a given location.

Respiratory viruses are present throughout the year, but levels typically increase in the fall and peak during the winter months (November through March).

A community’s category for the selected respiratory virus reflects how current levels compare to levels at that site during past respiratory virus seasons (not using the off-season). These levels are an indication of the level of respiratory virus activity in a community at a given time.

• A “very high” or “high” level means that the amount of virus circulating in your location is higher than usual for a typical respiratory virus season, and you should take precautions to protect yourself from COVID, flu, and RSV, such as those listed below.
• A “moderate” level means that levels are comparable to what may typically be detected during a COVID, flu, or RSV season. Moderate levels still indicate that the selected respiratory virus is present in your community activity, and you should consider preventative measures.
• A “low” level means that levels are lower than during a typical respiratory virus season and could indicate that there is little or no flu activity in your location. However, “low” or “moderate” levels following a period of “very low” levels could be an indication that activity is beginning to increase for the season, and you should prepare for levels to increase.
• A “very low” level means that levels for that respiratory virus are minimal or not measurable in a sample. This is typical during the respiratory virus off-season.

Wastewater levels should be interpreted alongside other established public health data (see the DHS respiratory dashboard) in order to provide a complete picture of respiratory disease activity in Wisconsin. Even when respiratory illness levels are low to moderate, it is important to keep up with prevention strategies. Follow these tips to help keep you and your loved ones healthy:

• Stay up-to-date with vaccines.
• Take steps for cleaner air, such as gathering outside or bringing in outside air.
• Wash your hands and practice good hygiene.
• Cover coughs and sneezes.
• Stay home if you are sick.

If you or people around you have risk factors for severe illness, consider additional precautions at any time such as wearing a mask and physical distancing.

We are still learning about wastewater levels and what they mean for communities. It is not possible to determine the exact number of people infected with a given virus based on wastewater levels alone. Therefore, it is more important to look at the category and the direction of wastewater levels in your location, region, or state, not the total number of gene copies in the results.

Can I compare results from one community with another?

We do not recommend comparing results between communities. Each location or sewershed’s wastewater is different. For example, some are impacted by industrial sources and varying amounts of rainwater which affects viral concentrations differently. It is better to use data from a single location, or use regional or statewide views, to see trends over time.