Wastewater monitoring dashboard: Technical notes

We calculate the EpiWeek signal by averaging all wastewater viral signals from 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.

As new data becomes available, we may update the signals of the latest reported EpiWeek.

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
• 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.

Calculation of viral load

Geographical aggregation to city, province, territory or nation is the average of all collection sites within the respective area. The average is adjusted for population coverage. This means that collection sites with higher populations have a larger impact on the overall value. The greater the population coverage, the more representative the wastewater signal is of the overall community disease burden. Higher population coverage also increases the probability of detecting local outbreaks. Sites with low population coverage (less than 25% of the population) are 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. Geographical aggregates of viral load are only reported when data is available for at least half of the reported population is not available. At the national level, we only report aggregated data when data is available for at least 75% of the reported population.

Calculation of activity level index

• High = 3
• Moderate = 2
• Low = 1
• Not detected = 0

Geographical aggregation of the activity level to city, province, territory or nation is the average activity level of all collection sites within the respective area. The average is adjusted for population coverage. The population-weighted average is assigned an activity level according to the following criteria:

• 0 = Not detected
• 1.5 – 0 = Low
• 2.5 – 1.5 = Moderate
• 3 – 2.5 = High

Mpox virus is found in wastewater in very low quantities, between 10 and 100 times lower than respiratory viruses. Consequently there is a high uncertainty in any single measure of the virus in wastewater. Therefore, we interpret signals across multiple EpiWeeks to reduce this uncertainty.

The NML tests for the presence and abundance of mpox virus in wastewater through 2 independent laboratory tests. A general mpox test is sensitive to all clades of mpox: for each sample, 6 replicates are tested across 2 different tests. Our map and dashboard reflects testing for general mpox and signals do not represent any specific clade. Additionally, a single clade I specific test is tested across 9 replicates per sample. A confirmed clade I detection will be communicated in text on the main results page of the dashboard.

The detection of mpox in wastewater is classified as:

• Consistent detection indicates that the mpox virus was detected in the wastewater for more than 2 weeks in the past 4 weeks.
• Intermittent detection indicates that the mpox virus was detected for 2 weeks in the past 4 weeks.
• No detection indicates that the mpox virus was not detected or detected for only 1 week in the past 4 weeks.
• No recent data indicates that less than 2 weeks of data was gathered to make an assessment about the presence of mpox virus in the wastewater in the past 4 weeks. This is usually due to delays in receiving samples.