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COVID-19 wastewater surveillance dashboard

Technical notes

This page has information about how we conduct wastewater testing and the limitations of the data. It also includes definitions for some of the scientific terms used in this dashboard.

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Methodology

Scientists across the country provide wastewater surveillance data through their provincial and territorial networks. We present data for all sites, including where an area is serviced by multiple wastewater treatment plants.

Scientists are still learning about how to detect and measure COVID-19 in wastewater. While there are different ways of doing this, the scientific community, including the Public Health Agency of Canada, is working together to build a standard that will help everyone understand, compare and share data about COVID-19 in wastewater.

We’ve compared the trends of wastewater signals when the same sites are tested by both the National Microbiology Laboratory and provincial and territorial networks. We found that the trends are broadly consistent across labs. Differences in the strength of the wastewater signals are mostly due to differences in processing methods.

We present COVID-19 viral load as a 7-day rolling average because high levels on a single day don’t show the broader trend. Our approach helps us to understand the overall trends while giving you better information to make your own health decisions.

We monitor the rise and fall of COVID-19 signals using a technique developed by the Ontario Ministry of the Environment, Conservation and Parks as a part of their provincial wastewater surveillance initiative. Briefly, the wastewater surveillance 7-day average data is broken into segments over time. The daily change in the viral signal is determined for each segment. Rises and falls of the wastewater signal are judged based on their consistency over time.

For more information, please refer to: Quantitative Trend Analysis of SARS-CoV-2 RNA in Municipal Wastewater Exemplified with Sewershed-Specific COVID-19 Clinical Case Counts.

Limitations

While wastewater surveillance offers many advantages, it does have some limitations.

The wastewater signal can be hidden by the composition of wastewater, which varies by community. For example, ground or surface water can make the COVID-19 wastewater signal stronger or weaker. This can be an issue during seasonal snow-melt and large rain events.

The wastewater signal can also be affected by:

We’re working with our partners to identify other issues with wastewater surveillance, and developing measures to reduce the effects.

Wastewater surveillance also detects people with or without symptoms. Considering the above limitations, we’re not sure how much virus is shed with each wave. For this reason we don’t recommend comparing wastewater surveillance data from different waves of COVID-19 to estimate the number of cases in a community.

Definitions

To learn more about wastewater surveillance, please refer to Harnessing the power of wastewater testing to detect COVID-19 outbreaks.

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