Coronavirus wastewater monitoring: new Omicron subvariants the most common variants in Finland

The results of wastewater monitoring carried out by the Finnish Institute for Health and Welfare (THL) indicate that BA.4/BA.5 subvariants of the Omicron coronavirus variant are now more common than the BA.2 subvariant in Finland. The relative proportion of mutations indicating subvariants BA.4 and BA.5 exceeded the proportion of the BA.2 subvariant in wastewaters at monitoring locations around midsummer. Around the same time (in week 25), BA.4/.5 subvariants also became the dominant virus in nationwide variant sequencing of that covers a sub-sample of laboratory-confirmed COVID-19 patient samples.

Wastewater monitoring investigates the total amount of coronavirus genetic material (RNA) and the proportions of different coronavirus variants in Finnish wastewater. The RNA number has remained at a high level in wastewater, which means that a large number of infections is still circulating among the population. As a major share of the infections are only verified by home testing, laboratory-confirmed cases do not reflect this situation.

As viruses multiply, mutations occur in their genome. Mutations are part of the natural evolution process of viruses. Separating Omicron BA.4 and BA.5 subvariants from each other in wastewater data is currently not possible due to the similarity of their genomes.

Latest subvariants first detected in May

Omicron BA.4/5 subvariants were detected in Finnish wastewaters for the first time in Kuopio on 8–9 May 2022. By the end May, these subvariants were already detected in the wastewaters of ten different monitoring locations. The most recently completed sequencing results are from wastewater samples collected in mid-July 2022, at which time the latest subvariants were dominant at all locations covered by THL’s wastewater monitoring.

Proportions of different SARS-CoV-2 virus variants in Finnish wastewater as found in the sequenced wastewater samples from monitored wastewater treatment plants between 1 January 2021 and 18 July 2022. The paler area represents the standard deviation from the results of different treatment plants. 

The first indications of Omicron BA.2 subvariant in Finnish wastewaters were found at wastewater treatment plants in Kuopio and Lappeenranta in samples collected on 2–3 January 2022. This subvariant remained dominant from early March until late June. In wastewater samples collected on 17–18 July 2022, subvariant BA.2 was only detected in Kuopio.

Further reading: Coronavirus variants in wastewater

Wastewater monitoring to be continued on a weekly basis but at fewer locations

The collection of samples for coronavirus wastewater monitoring will continue on a weekly basis, and after the summer break, the results are again reported once a week at 12 noon on Fridays.

In September 2022, the monitoring will be cut back, and the RNA number will only be examined in wastewater samples from 9 locations on a weekly basis. This will reduce the population coverage nationally from 50% to approx. 45%.

From 4–5 September 2022 on, no samples will be taken at Luotsimäki wastewater treatment plant in Pori, Paroinen plant in Hämeenlinna and Mäkikylä plant in Kouvola. Additionally, instead of weekly sampling, samples will only be collected once a month at Toikansuo plant in Lappeenranta and Alakorkalo plant in Rovaniemi.

At other monitored treatment plants, sample collection will continue at the current frequency for autumn 2022. From 4 September 2022, samples will be taken once a week at wastewater treatment plants in Espoo, Helsinki, Joensuu, Jyväskylä, Kuopio, Oulu, Tampere, Turku and Vaasa. Additionally, samples will be collected once a month in Rovaniemi, Lappeenranta, Pietarsaari and Seinäjoki. 

The coronavirus RNA numbers detected in the samples are updated in the weekly report on wastewater monitoring on THL’s website. The changes to the monitoring frequencies and locations will be visible in the weekly report for the first time on Friday 9 September 2022. Wastewater samples in which coronavirus RNA is detected are sent for sequencing in order to identify the coronavirus variants.

This week (7–8 August 2022), high coronavirus RNA numbers were again seen at participating wastewater treatment plants. While the rising trend of the last five measurements continued in Jyväskylä, the increase had levelled out in Kuopio and Tampere. Coronavirus RNA numbers had remained steadily high in Espoo, Rovaniemi, Turku and Vaasa, whereas they had started declining in wastewaters in Hämeenlinna, Joensuu, Lappeenranta and Oulu, and a declining trend has persisted in Helsinki and Kouvola.

The situational picture of the numbers of coronavirus observed by wastewater monitoring is independent of test activity, and it supports other epidemic indicators. 

Further information

• Further information on coronavirus variants in wastewater
• Weekly report on wastewater monitoring for coronavirus (in Finnish)
• Coronavirus variants 

Tarja Pitkänen
Chief Specialist
THL
tel. +358 29 524 6315
[email protected]

Anssi Lipponen 
Senior Researcher
THL
tel. +358 29 524 7962
[email protected]