Calgary and Edmonton COVID-19 wastewater readings trending downward

As the number of people hospitalized with COVID-19 in Alberta reached another record high, wastewater numbers in at least Edmonton and Calgary show a declining number of new COVID-19 infections.

The data from a dashboard created by the University of Calgary Centre for Informatics show the average amount of COVID-19 detected in wastewater has trended downwards since a peak on Jan. 11 in Calgary.

COVID-19 tracking by wastewater can work much like a forecast for numbers such as test positivity, hospitalizations and deaths further down the line.

"A weather report is a great analogy," says U of C geomicrobiology professor Casey Hubert who works with the group collecting samples in Calgary.

"We haven't had 10 points in a row that evolves on down, down, down. But if we look at the last 10 or so points, we can see that the trend is down."

The data from wastewater is considered a better indicator of how a city is doing at this point in the pandemic, particularly at a time when the province is rationing tests to high-risk groups and many people who have contracted COVID-19 are not showing up on provincial case totals.

Wastewater testing measures COVID-19 levels across an entire city and officials can get a big picture perspective sooner than relying on just nasal testing.

Looking at the charts themselves can reveal trends in the data, but Hubert warns not to focus too much on the individual readings on a given day and to focus on the average line, which averages over three sample days or a week.

Here is the chart for Calgary.

Numbers in Edmonton show a similar downward trend having peaked on Jan. 3.

The seven-day average of test positivity rates for the whole province of Alberta peaked at 39.6 per cent on Jan. 12 but that number remains high at 37.2 per cent on Jan. 27.

The situation in Alberta's medium-sized cities shows a more mixed picture. Lethbridge until recently was showing a concerning trend — the average line pointing straight up — but with today's most recent reporting from Jan. 25, we may be seeing signs of a turnaround.

The chart for Fort McMurray, with data only going back to late 2021, is showing increased prevalence of the virus since the start of the year. 

The viral trend in Medicine Hat is pointing upwards, but hasn't yet reached levels seen during the Delta wave.

Grande Prairie has so far been spared an Omicron spike seen in many other parts of the world.

Due to methodological and wastewater system differences, it is also important not to compare the raw numbers from one city to another on a given date.

"For example, in Calgary, the stormwater and the rainwater are separated from the sewer and in Edmonton it's not. Just right there you can see that to be a tricky comparison," Hubert says.

"What we do is just compare one city to itself over time."

Trending down but still high

While wastewater numbers are showing promise for the coming weeks, Dan Gregson, an infectious disease physician and medical microbiologist at the University of Calgary, cautions those who might see a rosy picture.

He points out that virus levels in wastewater now are still higher than they were in December when Omicron cases began to skyrocket in Alberta.

"It doesn't really tell you how many people are going out of the hospital and it doesn't tell you when we can ease restrictions. It tells you that things are going to drop but we're not really back to baseline," he says.

The data, which was updated Friday, contains samples taken as recent as Jan. 25. The dashboard, while created by the U of C, contains data collected from communities across Alberta and is a collaboration of academics, researchers and governments.

Many sites use different collection and viral counting methods. The y axis in the Calgary chart shows the raw number RNA molecules of SARS-CoV-2 — the virus that causes COVID-19 — collected in each sample, while others show amount of RNA per millilitre of water.

Once samples are collected from water treatment plants, researchers perform a polymorase chain reaction (PCR) test — the same test used on samples collected from people's noses.

"Basically what we do for COVID-19 is we do PCR testing of the whole city," Hubert says.