First, each point on the graph represents one sample taken in the study. When reporting viral load data, the percent change reported compares the average of the 3 individual samples taken during the week, to the average of the 3 samples from the week before. Therefore, when looking at week-over-week changes in the graphs, one should compare the 3 most recent points on the graph, to the 3 points before.

With increasing vaccination rates, we are expecting to see a decrease in new cases going forward (as is already observed in older age groups that have been vaccinated first). That decrease in newly infected people is then also expected to result in a reduction in viral traces in the wastewater. For the time being, however, while Saskatchewan has one of the lowest rates of fully vaccinated residents in Canada (around 58% fully vaccinated, as of August 25), epidemiologists estimate that a vaccination rate upwards of 75% is required to achieve herd immunity. Thus, we do not expect to see the impact of vaccinations in wastewater just yet.

In the best case, a vaccinated person would be protected and not get infected with the virus in the first place, thereby preventing any shedding altogether. With the arrival of the variants of concern (VOCs) in Saskatchewan, epidemiologists see an increasing number of breakthrough infections in vaccinated individuals. These are often asymptomatic or have relatively mild symptoms but can nonetheless shed the virus and be infectious.

The mRNA vaccines (Moderna, Pfizer-BioNTech) are based on sequences of the S gene (i.e., the spike protein that allows the virus to enter host cells). The viral RNA signal we are tracking in the wastewater is for the N gene (i.e., the nucleocapsid shell of the virus). These are two entirely different sequences without cross-reactivity. The N gene is also widely used in very similar clinical diagnostic tests for the coronavirus that are performed on individual swab samples. These tests – like the one we use for wastewater – will not identify a patient who was recently vaccinated with one of the mRNA vaccines as positive for the coronavirus. Please note that this can be different for rapid testing kits that are based on antibodies.

No. The risks of becoming infected through wastewater are negligible compared to the commonly known routes of transmission. Additionally, effluents of Saskatoon’s wastewater undergo a rigorous disinfection procedure. Similarly, our drinking water is thoroughly treated resulting in safe water for all residents.

Depending on stage of infection, individuals infected with SARS-CoV-2 may shed greater or lesser amounts of the virus through their feces. During the exponential phase of an outbreak, many individuals are in a similar stage of infection, and increases in the viral signal in wastewater are often directly proportional to new case numbers, as was observed in October and November 2020 in Saskatoon. Under different conditions, individuals are typically in various stages of infection and recovery, and the proportionality decreases.

We provide weekly reports to the City of Saskatoon, the Saskatchewan Health Authority, and as well as the Public Health Agency of Canada.

Data from other jurisdictions show that fluctuations in the wastewater collection and treatment system, e.g., influx of water and road salts during snowmelt, can have a considerable impact on the viral signal in wastewater. In Saskatoon, domestic sewers and storm sewers are separate, and average daily flows, water temperatures, dissolved and particulate matter, as well as pH, are remarkably stable throughout the year. All of these factors contribute to the exceptionally high predictive value of wastewater-based surveillance in Saskatoon.

It has been proposed that individual stool samples could be used to test whether a patient tests positive for SARS-CoV-2. However, some studies indicate that not all patients will consistently shed the virus through their feces. Thus, nasopharyngeal swab testing is still considered the gold standard for individual patient testing. Thus, while wastewater data shows the trend of overall infected cases at the municipal level, it is not a diagnosis tool for individuals. Anyone who thinks he or she might have COVID or who has been exposed to someone with COVID should follow Saskatchewan Health Authority instructions and get a swab test.

As we announced at the end of November 2021, the protocol used for Saskatoon was changed to be more in line with the one being used for the other two cities in the study:

“Please note: The wastewater surveillance team has used a test protocol based on the supernatant fraction of primary influent for their surveillance efforts in Saskatoon. Due to the higher throughput, samples from all other monitored locations in Saskatchewan were processed using a whole raw influent protocol. We have collected data for the Saskatoon WWTP using both methods since May 2021, and the methods provide essentially the same trends. Furthermore, the whole raw influent protocol offers slightly greater sensitivity. Therefore, beginning with the next report, the team will begin generating and reporting data and trends using the whole raw influent protocol only.”

As the announcement states, much data was collected using both methods, and both methods showed essentially the same trends. Since it is the rate of change which is key to interpreting this data, the absolute viral load numbers are not directly meaningful, which is why we were able to make the switch relatively seamlessly.

Overall viral loads and variant-specific viral loads are determined using separate sets of tests with different sensitivities. Thus, viral loads should not be directly compared.