Since late February, KWR has been measuring the concentration of genetic material of the coronavirus (SARS-COV-2 RNA) in the sewage water of seven cities, Schiphol Airport, and Terschelling (island). That concentration reflects the level of contamination in the population of the city connected to the sewage system under investigation. This page shows the current analysis of the sewage samples taken on Wednesday, 29 July.
By conducting regular measurements, we have established graphs for several cities since the start of the corona crisis. The curves correlate with the number of registered cases with the Dutch Municipal Health Services (GGD) and the number of hospital admissions. It is striking that the sewage water curve ‘reacts’ about a week before the other two measures. This is not very surprising because people infected with the coronavirus have no symptoms in the first few days but they do excrete the virus into the sewers. This means that the sewers can be used as an early warning system.
The curves started to rise steeply starting in early March. There was a peak in most locations in late March, followed by the start of a downward trend in early April. In recent weeks, the concentration of virus particles has fallen below the detection threshold in a number of cities. This means that the sophisticated measuring equipment at KWR cannot detect SARS-COV-2 RNA in the sewage sample. The rising curves of the past three to four weeks have turned into a slight decrease in most locations last week.
Sewage samples from 29 July
The upward trend in Amsterdam this week shows a slight decline.
The rising trend in Utrecht has turned into a sharp decline.
In Amersfoort the concentration has remained the same over the past four weeks.
In The Hague a slight decrease in concentration this week, after a slight increase last week.
In Tilburg a slight increase in concentration this week, after a slight decrease last week.
At the Rotterdam Dokhaven location, the signal remains almost the same (sampling 19 July).
In the other locations measured by the KWR, we found (sampling 15 July):
- In Apeldoorn and Terschelling: Still below the detection limit.
- Franeker: Slight increase compared to last week, but still in the same range.
- Schiphol: Slight decrease compared to last week.
A close eye on the graphs
We are now at the end of the first wave. The increase in the SARS-CoV-2 RNA concentration in sewers at the beginning of the first COVID-19 wave proved to be indicative at that stage of a later increase in the number of hospital admissions. It seems reasonable to assume that the same pattern will be seen in a second wave. However, there is a difference between the first and second wave. In the first, the virus (RNA) was not present at all beforehand. We are now at the tail end of the first wave and the virus (RNA) is still present in the sewers. That is why it is now important to keep a close eye on any increase.
By looking at whether the increase continues, and by comparing the sewage data on the national dashboard with the other sources of information: the cases reported to the municipal health services and hospital admissions. Gertjan Medema, the principal microbiologist at KWR: ‘None of these sources of data are perfect but, together, they do provide a more complete picture of how the virus is circulating. That is the work of the municipal health services.’
So far, the national corona dashboard does not show similar curves from locations in The Netherlands (including all provincial capitals) measured in recent months.