An increase in coronavirus was seen in samples taken at several wastewater treatment plants last month, the Institute for Health and Welfare (THL) announced on Tuesday.
The virus was detected in wastewater treatment plants of five cities where confirmed cases of the disease have not been seen in some time.
In samples taken last month, coronavirus was found in the wastewater plants in the cities of Jyväskylä, Hämeenlinna, Kuopio, Lappeenranta and Kouvola. Follow-up samples were collected on 23-24 and 30-31 August.
Because of the disparity between the presence of the virus in the water and lack of recent confirmed Covid cases, THL warned there may be infected individuals unknowingly spreading the disease to others in those communities.
The agency noted that no coronavirus infections had been detected in the Kouvola area for five weeks before water samples were examined.
"The fact that the virus is present in effluent where no infections have been detected is a warning sign. There may be unidentified infected people in the area and the virus can circulate within the area's population," THL's specialist researcher Tarja Pitkänen said in an agency statement.
Coronavirus infections were most recently confirmed in Jyväskylä and Hämeenlinna towards the end of July, while since last spring, there were a few isolated cases seen in Kuopio and Lappeenranta in August.
The health agency said that coronavirus was detected in the wastewater treatment plants of eight hospital districts on 23-24 August.
THL said it plans to increase wastewater testing, in an aim to more effectively measure the spread of coronavirus infections. The results can also help authorities predict and prepare for a spike in infections up to one week in advance.
The coronavirus water testing programme first started as trials in the cities of Helsinki, Turku, Tampere, Kuopio and Oulu, later extending to other areas.
The agency monitors the presence of coronavirus in wastewater by measuring the quantity of RNA, the virus's genetic mass, in samples collected from treatment plants, as RNA can be used to study the prevalence of coronavirus in the population.