University of Tartu Professor Tanel Tenson, who heads up the study, conducted on a weekly basis, said: "In the south, the infection has spread rather extensively from larger cities to many smaller places. We can expect the number of the infected to increase in that region."

Tenson, who is Professor of Technology of Antimicrobial Compounds, noted that though South Estonia stands out, this week has brought a slight rise in virus amounts almost everywhere.

The Health Board (Terviseamet) uses the weekly surveys to monitor changes in the outbreak dynamics, and to discover hidden outbreaks. The studies provide early information for estimating the spread of the virus, ahead of clinical cases being detected. The Health Board keeps regularly informed of the results.

A week ago, Ida-Viru County stood out in having lower levels of coronavirus traces than average nationwide. This week, samples collected there include some where virus amounts exceed the Estonian average, the university said.

The University of Tartu team headed by Tanel Tenson collects its samples at the beginning of each week, in population centers with more than 10,000 inhabitants (which number around 15), and sometimes in smaller population centers too, via spot checks.

In the larger cities the samples passing through treatment plants over a 24-hour period give a reliable overview of the infection level in the city, the university says, while the spot samples in other locations need to be analyzed over a lengthier time-frame to get an accurate picture.

The University of Tartu cooperates with the Estonian Environmental Research Centre and water companies nationwide.

Most water providers are municipality-owned, the major exception being the private sector Tallinn Vesi, serving the capital.

Waste-water refers to sewage, rather than washing machine outflow, car-washing and other surface run-off.

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