Coronavirus – The crux of the wastewater analysis – News


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Since February, the federal government has been measuring the viral load from Corona in over 100 sewage treatment plants. But the analyzes are only useful to a limited extent.

We commute on the train without a mask, eat in the restaurant without showing the certificate. A year ago, that still sounded like a dream of the future. But even if the corona virus is no longer so present, the federal government collects data.

In addition to case numbers or line samples in various population groups, the Federal Office of Public Health FOPH also relies on data from wastewater. “Due to the changed test behavior of the population, an increased number of unreported cases can be assumed, while the wastewater monitoring reflects the infection process independently of the test behavior of the population,” writes the BAG on request.

Wastewater analysis does not allow forecasts

The analysis of the wastewater provides information about where the virus is still circulating or is circulating again and whether measures are necessary. But the evaluation also has limits. For example, no differences can be identified between those who have recovered who are still excreting “Covid material” and those who are newly infected. The sample also does not allow a forecast because it is a snapshot.

Since February 2022, more than 100 sewage treatment plants have been providing the BAG with their analyses. These are public. This shows: In which sewage treatment plant has the viral load increased and where has it decreased?

The following map shows this development. However, it is not possible to trace how many communities each sewage treatment plant serves. For the sake of simplicity, we have only colored the municipality in which the facility is located on the map.

However, the data from the BAG only allow limited comparisons. Because the 100 or so sewage treatment plants do not represent all regions and communities in Switzerland. They cover around 70 percent of the Swiss population. The BAG finds this to be sufficient: “The selection criteria for the wastewater treatment plants (ARA) included the size of the catchment area, the representation of at least one ARA per canton and the inclusion of large tourist areas.” The BAG does not plan to sample more ARA in the future. The current monitoring seems to be representative for the whole of Switzerland.

It is not possible to compare the systems

Not only the geographical coverage is difficult, but also the comparison between the different areas. Because the data collected only relates to the respective system. The percentage of exposure to coronaviruses in the current week depends on the highest load on the system that has ever been measured. However, since this value can differ from plant to plant, a comparison between the sewage treatment plants is not possible. In addition, the systems do not all do their evaluations on the same day.

Legend:

The viral load in the sewage treatment plant in Altdorf has stagnated in the last seven days.

KEYSTONE/Martin Ruetschi

The FOPH is aware that the absolute values ​​of the viral load are difficult to compare. But: “For epidemiological monitoring, the trend development over time is of particular interest, i.e. whether the viral load in the relevant catchment area is increasing, decreasing or stagnating. The relative viral load has proven to be very effective for this.”

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