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Publication - Research and analysis

Coronavirus (COVID-19): modelling the epidemic (issue no. 29)

Published
3 December 2020
From
Director-General Health and Social Care
Directorate
Constitution Directorate
Part of
Coronavirus (COVID-19) in Scotland
ISBN
9781800044029

Latest findings in modelling the COVID-19 epidemic in Scotland, both in terms of the spread of the disease through the population (epidemiological modelling) and of the demands it will place on the system, for example in terms of health care requirement.

This document is part of a collection

View supporting documents Supporting documents

What can analysis of wastewater samples tell us about local outbreaks of Covid-19 infection?

Samples from Waste Water Treatment Works (WWTW) in Scotland have been analysed by the Scottish Environment Protection Agency (SEPA) to detect fragments of SARS-Cov-2 virus RNA in waste water. This is reported from lab analysis as gene copies per litre. The reported levels of SARS-Cov-2 found have been compared to trends in confirmed Covid-19 cases in the surrounding areas.

Figure 14 shows that the level of gene copies recorded at Carbarns WWTW and Shieldhall WWTW increased as the number of positive cases increased in North Lanarkshire and Glasgow City, respectively, from September to October. The concentration levels then came down as the number of confirmed cases in these locations reduced over the past month.

Analysis of Covid-19 viral concentrations in comparison to recorded positive tests shows similar trends to that in Figure 14 for a number of other outbreaks around Scotland. A comparison of viral RNA levels at each site and confirmed positive tests in the surrounding area is shown in Figure 15. While this approach may not provide an early indication of a developing outbreak, it gives us confidence that the outbreak has indeed been contained and we are not missing a substantial number of asymptomatic viral spreaders.

Figure 14: Temporal trend of the recorded weekly rates of positive Covid-19 tests in North Lanarkshire and Glasgow City from June to December 2020, and the normalised viral RNA levels at Carbarns WWTW and Shieldhall WWTW. Viral RNA levels have been normalised by flow rate, and the population of the WWTW catchment area, and an average has been taken over the samples reported each week. In this plot the red circles (weekly cases per 100,000) are scaled relative to the maximum sewage viral levels found at each WWTW. Further information is available in the technical annex of issue 26.
Two scatter plots showing the temporal trend of the recorded weekly rates of positive Covid-19 tests in North Lanarkshire (top) and Glasgow City (bottom) from June to December 2020, and the normalised viral RNA levels at Carbarns WWTW (top) and Shieldhall WWTW (bottom).
Figure 15: For each WWTW site where samples have been taken, the normalised viral RNA levels against the weekly rates of positive Covid-19 tests in the closest local authority from June to December 2020. Clusters of points in a diagonal line from the lower left corner to the upper right shows evidence of positive correlation between viral RNA levels and cases. Large population centres seem to have more favourable correlation, for example Carbarns. Local authorities with a more dispersed population, where the waste water is measured at relatively small sewage works and cases are recorded at local authority level, seem to show less evidence of correlation, for example Hawick in the Scottish Borders.
A series of scatter plots for each WWTW site where samples have been taken, showing the normalised viral RNA levels against the weekly rates of positive Covid19 tests per 100K, in the closest local authority from June to December 2020

Contact

Email: modellingcoronavirus@gov.scot

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