Respiratory viruses: Wastewater monitoring 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.

On this page

• Methodology
• Limitations
• Definitions

Methodology

Scientists across the country provide wastewater monitoring data through their provincial and territorial networks. To detect respiratory viruses at the community or institutional level, samples are taken at a central collection point, such as wastewater treatment plants or pumping stations. This method only captures the presence of respiratory viruses in the community or institution. It can’t be used to identify single cases or households.

Scientists continue to improve methods for detecting and measuring COVID-19, flu and RSV in wastewater. 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 respiratory viruses in wastewater. The results shown on this page were obtained by PCR testing and genomic sequencing.

Scientists use genomic sequencing to decipher the different genetic fragments of the virus found in the wastewater samples. Once the sequencing reaction is complete, they analyze the sequenced pieces using special software. These programs provide information on the variants and the relative amount of each variant detected in a wastewater sample.

We’ve compared wastewater signal trends 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, flu and RSV wastewater viral load testing as a population-weighted 7-day rolling average. This is 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. Generally, we test sites twice weekly. Exceptions are Alberton and Winnipeg which are tested 1 and 5 times per week, respectively.

How the EpiWeek signal is calculated

The EpiWeek signal is calculated by averaging all wastewater viral signals collected within that EpiWeek. Wastewater signals are aggregated to the EpiWeek for all collection sites within the same region as these sites can have different sampling schedules.

How the geographical aggregation is conducted

Geographical aggregation to city or province level is the average of all collection sites within the city or province, adjusted for population coverage. Collection sites that cover more population have a larger impact on the overall value for the city or province. The greater the population coverage the more representative the wastewater signal of the overall community disease burden and the greater the probability of detecting local outbreaks. Any sites with a low population coverage, i.e., less than 25% of the population have been indicated below the graphs.

In each graph the grey shaded area shows the minimum and maximum wastewater signal of each EpiWeek since the beginning of the surveillance period, based on available data.

How the wastewater activity level index is calculated

To provide more context to COVID-19, flu, and RSV signals, we’ve developed an activity level index. It’s based on a combination of the signal level and signal trend metrics. We need at least 12 months of data, excluding non-detects, to calculate the signal level and trend metrics.

• High activity level means:
  • viral signals are high compared to values over the past year, or
  • viral signals are moderate compared to values over the past year, and the recent trend is increasing
• Moderate activity level means:
  • viral signals are moderate compared to values over the past year, and the recent trend is decreasing or stable, or
  • viral signals are low compared to values over the past year and the recent trend is increasing
• Low activity level means:
  • viral signals are low compared to values over the past year, and the recent trend is decreasing or stable, or
  • viral signals are below the threshold for detection, and the recent trend is increasing
• No detection activity level means:
  • viral signals are below the threshold for detection and the recent trend is decreasing or stable

Calculation of signal level metric

The signal level metric describes the most recent COVID-19, flu and RSV viral loads compared to values over the past year. For each sampling location, we use viral loads from all samples collected 1 year before the most recent sampling period, to:

• calculate the 25th and 75th percentiles, and
• establish lower and upper cutoff values

Values below the 25th percentile are classified as low and values above the 75th percentile are classified as high. The rest are classified as moderate. Sites are marked as new if there are less than 12 months of data, excluding non-detects.

Calculation of signal trend metric

We monitor the rise and fall of COVID-19, flu and RSV signals using a technique developed by the Ontario government. The wastewater monitoring 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

The signal trend metric describes how wastewater viral loads are changing based on the previous 5 weeks.

• Increasing is a statistically significant increase in the wastewater signal.
• 'Stable' is either a steady signal or an insignificant decrease in the wastewater signal.
• Decreasing is a statistically significant decrease in the wastewater signal.

Limitations

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

The accuracy of the wastewater signal can be affected by various factors, including the composition of wastewater, which varies by community. For example, ground or surface water can make the 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:

• industrial flow into the wastewater collection system
• sand and salt to roads in winter
• the temperature of the collection system
• the method used for collecting the sample
• the diversity of variants in a sample
• the presence of similar mutations found in different variants

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

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 monitoring data from different waves of respiratory viruses to estimate the number of cases in a community.

Definitions

• The 7-day average is generated by averaging the levels from a given day with the 6 previous days. The average is termed “rolling” as it changes each day.
• Copies per mL is the number of copies of the target RNA found in a milliliter (mL) of raw wastewater by the specific wastewater treatment facility.
• Viral load is the concentration of SARS-CoV-2, Influenza, or RSV genetic material present in a sample of wastewater. We present this in the dashboard as copies of SARS-CoV-2, Influenza, or RSV genetic material per milliliter (mL) of wastewater.
• Wastewater signal is a measure of the level of virus in wastewater that identifies increasing, stable or decreasing number of virus particles in wastewater.
• 25th percentile is also called the first quartile. 25% of the data falls below this level.
• 75th percentile is also called the third quartile. 75% of the data falls below this level.
• Non-detect is when the detection limit of the respiratory virus was not reached during testing of the wastewater sample.
• Epidemiological week (EpiWeek) is an approach to counting weeks that allow for comparison of data between years. For the EpiWeek appearing on the x-axis of the graphs we have used Sunday as the first day of the week.

To learn more about wastewater monitoring, please refer to:

• Harnessing the power of wastewater testing to detect COVID-19 outbreaks

Data changes