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Wastewater CV-19 Levels, June 1

By June 6, 2022Commentary

This has turned out to be somewhat disappointing, as it appears that methodological issues limit the value in identifying trend with any certainty, but it is something worth looking at.  The relationship between cases and wastewater levels is highly variable in different regions of the state, and maybe that reflects different testing levels, but seems very odd.

Dave’s notes:

  1. We last looked at correlations between wastewater and Covid case levels in Minnesota on May 26 here:  Since that post the University of Minnesota has updated virtually every single the data point for the entire series. They do not publish any documentation on this data set, but it is difficult to understand why the data is being revised so long after initial publication. If It necessary for the data to be revised so long after the fact, then it destroys the usefulness of this data set in understanding what is happening with Covid in real time (which is the main point of this exercise).
  2. The University of Minnesota publishes summary data of Covid sampling of wastewater in Minnesota here: The U of M is monitoring wastewater plants for the presence of Covid indicators. They state that the amount of Covid detected in wastewater is predictive of clinical cases of Covid 1 to 2 weeks later. They also state that they are studying whether their metrics correlate to hospitalizations.
  3. In the following charts we are plotting the U of M metric “Mean Weighted Average (N and O genes)” of virus copies per liter (blue curves) along with the newly reported covid cases per 10,000 residents in each region (red curves). We have yet to find any documentation describing exactly what the mean weighted average represents. For the purposes of this post it is assumed that the weighting process accounts for the differing populations in each region. For this reason we have chosen to plot Covid cases as the cases per 10k residents, rather than simply number of Covid cases per region.
  4. It is important to note that the U of M states that they changed their methodology on 3/15/2022. This means that the older data they publish, from 1/1/2021 to 3/15/2022, is not directly equivalent with the post-3/15/2022 data. We are only publishing the newer data in these charts. Also, the mean weighted metric we are displaying only starts on 4/6/2022. Some of the 4/3/2022 data points available last time have been deleted by the U of M, so they have been removed from the charts as well.
  5. The cases per 10,000 plotted in the charts are not the cases publishes by the U of M. Rather, we are taking cases per week per county from the Minnesota Department of Health Weekly Report archive,, which includes cumulative cases per county each week. From this data we calculate cases per week, and then cases per week per region per 10,000 population.
  6. The regions are defined by the graphic on the U of M wastewater surveillance web page, which we have reproduced in Fig. 8.

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