Why scientists say wastewater surveillance needs to continue, despite low COVID-19 levels

Despite a majority of wastewater sites across the country reporting some of the lowest levels of COVID-19 since analysis began, some researchers say this type of surveillance should continue so Canada can be prepared for the next pandemic.

Wastewater surveillance gained prominence in recent years by providing health officials with a summary of the degree of disease among Canadians during peaks in the pandemic. But its value beyond the coronavirus was quickly realized as a crucial public health tool that could help protect people from future harmful infections.

Of the 39 sites tracked by Canada's COVID-19 wastewater surveillance dashboard, recent data shows that about 60 per cent are reporting decreasing levels of the virus. The rest of the sites are stable or slightly lower than their previous readings.

"In the past, I would say, several weeks, we've seen a relatively consistent downward trend," said Dr. Guillaume Poliquin, vice-president of the National Microbiology Laboratory, part of the Public Health Agency of Canada (PHAC).

Wastewater reporting began in 2020, not long after the pandemic was declared by the World Health Organization, with more sites added as it continued.

Even though the amount of the virus at many of PHAC's sites appears to be at its lowest point since reporting began, Poliquin said it comes with a caveat.

"The techniques that are used, how accurate they are, how sensitive they are, has generally been getting better over time," he said.

"Monitoring has changed over time. It's not all the time comparable, but the trend is certainly reassuring."

It became apparent during the pandemic that COVID-19 levels will fluctuate based on the time of year. Disease spread tends to be lower in the warm summer months, with one reason being that people spend more time outdoors.

At the same time, 80.5 per cent of Canadians are fully vaccinated against the virus and more than 4.6 million have been infected, according to PHAC data — although its website notes that due to changes in testing, the number of cases might actually be higher.

The practice of gathering wastewater from different communities and analyzing it for COVID-19 is a public health tool that researchers have said is useful to predict disease trends and support decisions around public health policies. But it's also a strong indicator of what's to come, as experts can see whether a new strain of the virus has emerged.

Initially, 21 per cent of Canada's population was being monitored, but it's now grown to more than 60 per cent across the country, Poliquin said.

Survey results published last month in The Lancet journal show that Canada was one of the top countries, among 43 that participated, whose wastewater surveillance efforts covered the majority of the population. The article goes on to emphasize how significant this sort of testing tool has proven to track current and emerging health threats.

Although the COVID-19 pandemic was recently downgraded from global emergency status, continued wastewater surveillance is still very important, said Eric Arts, Canada Research Chair in viral pathogenesis and control at Western University in London, Ont.