Updates to COVID-19 Wastewater Surveillance Report – April 2023


13 April 2023

Public Health Ontario (PHO) has updated its COVID-19 Wastewater Surveillance in Ontario report to align with the Government of Ontario’s changes to the Wastewater Surveillance Initiative's sampling location strategy. At the same time, PHO has updated the statistical software used to run the analysis to a more flexible and sustainable system as well as changed the population source used to weight the signals from the entire public health unit population to sewershed populations.

About Wastewater Surveillance
Since the onset of the COVID-19 pandemic, surveillance has been critical to tracking and preventing the spread of the SARS-CoV-2 virus. Wastewater surveillance can be used by public health experts to monitor diseases within communities, including COVID-19 and other viruses like polio, measles, and hepatitis A. This is done by testing community wastewater for the presence of the viruses from infected individuals — called the wastewater signal. The wastewater signal can be tracked and analyzed for population-level trends over time and are a useful component of COVID-19 surveillance strategy, particularly for monitoring and to complement other existing COVID-19 surveillance indicators such as: percent positivity, hospitalizations and deaths. To learn more about how wastewater surveillance is conducted, please visit Tell Me More: Wastewater Surveillance.

What is changing?
As of April 1, 2023, the Government of Ontario’s Wastewater Surveillance Initiative updated the locations where wastewater samples are collected. The initiative launched a strategic sampling network that represents data from each public health unit area across Ontario. This network provides a more flexible, effective approach to sampling as it improves frequency and reduces duplication of sample collection to enhance trend analysis and provide early warning indicators to support public health and health care decision making. For more information about the initiative, contact the Ministry of Environment, Conservation and Parks.

PHO has also been completing quality improvement analyses and evaluations that have informed some additional analytical changes, including:

  • Updated the statistical software used to run the analysis for its report. This new system provides PHO with a more flexible and sustainable system to enhance COVID-19 wastewater reporting in Ontario.
  • Changed the population source used to weight the wastewater signals from public health unit (PHU) populations to sewershed populations.  A sewershed population refers to the number of people whose wastewater goes through a particular sewage system, compared to a PHU population, which refers to the number of people living in that specific region. In the analysis, weighing the wastewater signal refers to giving a value to each individual site or sewersheds so that it accurately reflects its relative contribution to the overall amount of wastewater sampled in the region.

Why do the graphs look different from the last report?
The graphs may look less smooth or the curves may change slightly due to the changes describe above, specifically:

  • Updates to the Government of Ontario’s Wastewater Surveillance Initiative: The initiative now uses a strategic sampling network strategy for collecting wastewater samples across each public health unit region, resulting in a change in the number of sample collection sites used in the analysis.
  • Changes to PHO’s statistical software used to run the analysis for its report (from STATA to R): The new software uses 14-day knots compared to 10-day knots used in the previous analytical system. A knot is a point where two different segments of a curve are connected, and the curve changes direction or slope at that point. They are used to fit a more flexible curve that can better capture nonlinear relationships in the data. Think of it like connecting pieces of a puzzle together to form a better overall picture. The number and location of knots are important to avoid adding too much complexity (overfitting) or over simplifying (underfitting) the data.
  • Change the population source used to weight the wastewater signals from public health unit (PHU) populations to sewershed populations (defined above) may result in slight changes in magnitude (lower or higher) of the wastewater signals. This will vary by region depending on population density.

Why might past data in the graphs change?
The curve in the graph represents the currently available estimates for the wastewater signal in that region. It is calculated using a statistical approach that uses data from the entire time period presented in the graph and estimates values for individual days where data may not be available yet. Therefore, past estimates may be under- or over-estimated and may change when new data become available, previously reported data are updated, or the analytical approach is changed.

For more information about PHO’s COVID-19 surveillance data and reports, please visit our COVID-19 Data and Surveillance webpage.

If you have questions about PHO’s COVID-19 wastewater surveillance reporting, please email healthprotection@oahpp.ca.

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Published 13 April 2023