How do I interpret this data?

Rather than looking at one data point, we recommend monitoring trends over time. Our analysis suggests that the concentrations of viral genes are highly correlated to clinical incidence rates in the sewersheds.

How should I read these results?

Results may be filtered by location to view quantities of different viruses in wastewater. Each location includes the viral copies per gram of dry solid waste. Points are raw data, while the line data in the chart have been smoothed, where measurements from nearby dates have been combined to reduce day-to-day variability and increase accuracy. Specifically, we take the 5 samples centered around a date and report the average after excluding the highest and lowest values. This processing minimizes the influence of outlier measurements and makes it easier to visualize trends in the data.

If the target is not detected in the sample, the concentration is reported as 0 c/g. There may still be very low amounts of the target present that are below the detection limits of our procedure. When a sample was not tested for a target, the concentration for that target is reported as “-”. Read more about variability in wastewater data.

What does “copies per gram” mean?

Results reported as viral copies (e.g. SARS-Cov-2) per gram (dry weight) of wastewater solids tells you how many copies of the virus were found in a gram of solid waste from a location. This data point can be used as an indicator of how many people in a sewershed (the geographical area served by a particular treatment plant) have the virus. Over time you will see this value rise and fall, indicative of more or less clinical cases in the population served by wastewater treatment plants.

What is PMMoV normalization?

In addition to human pathogens, we also measure an extremely common, harmless plant virus that is consumed when people eat. It is called pepper mild mottle virus (or PMMoV). By measuring the concentration of PMMoV genetic markers in wastewater solids, we can account for changes to the “fecal strength” of that sample. For example, heavy rain that drains into a wastewater system can dilute the strength of a particular day’s sample. This approach allows for comparison of results from day to day and between wastewater treatment plants.

Our analyses suggest that both the concentrations of the viral genes measured as copies per gram of solid waste and those concentrations normalized by PMMoV are proportional to laboratory confirmed viral infection incidence rates in the sewersheds. Further, in this study we have demonstrated that the relationship between these wastewater measures and COVID-19 incidence rates is consistent between sewersheds. That means a higher wastewater value at one plant compared to another suggests COVID-19 and other virus incidence rates are higher in that plant's sewershed compared to the others.

References