4 years later, COVID remains a year-round threat. Here's why this virus isn't seasonal quite yet

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A cursory glance at Canada's wastewater trends for COVID-19 reveals a messy, unpredictable picture: Viral loads ebb and flow all throughout the year, at different times, in different cities.

While SARS-CoV-2 is now a familiar threat, the virus isn't neatly seasonal. It still circulates year-round, humming in the background. And for the fifth year in a row, some scientists are bracing for the possibility of a small summer wave.

That reality might come as a surprise to anyone who hoped this virus would quickly join the typical colder-weather cold and flu season, offering a break from COVID infections over the warmer months. But we're not quite there yet.

"When you look at the other four coronaviruses — the cause of 25 per cent of our common colds — they do have this really stark seasonality," said infectious diseases specialist Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. "But we don't know how long it took for them to settle into that pattern."

SARS-CoV-2, on the other hand, is still in its infancy. And its spike protein, which allows the virus to penetrate our cells and cause infections, keeps mutating at a brisk pace.

"This is a virus that was never known to infect humans before 2019, so it's still a lot of evolutionary pressure, especially with the immunity that people have developed," Adalja said.

Close watchers of SARS-CoV-2's ongoing evolution are tracking several new variants, all growing more dominant in recent months. 

The JN.1 group remains the dominant form of the virus in Canada, while KP.2 and KP.3 — among the lineages nicknamed "FLiRT" by some scientists, after the technical names for specific genetic mutations — and LB.1 are all showing signs of growth, Public Health Agency of Canada data shows (PHAC). 

All of them are offshoots of Omicron, the variant that sparked a massive wave of infections midway through the pandemic. This still-circulating family of viruses remains more contagious than earlier forms, with spike protein mutations that help bypass the protection offered by vaccines or prior infections — ensuring people can get reinfected over and over.

"We've had nothing but Omicron for about two years," said Adalja. "That lineage is still trying to find the optimal combination to infect people, and there's always going to be some evolution going on.… It's still at a high enough pace that seasonality is not quite as predictable as people would like it to be."

While the results of nationwide tests suggest low circulation of common respiratory viruses such as influenza and respiratory syncytial virus in recent weeks, SARS-CoV-2 levels rose over several weeks until late May, says the most recent PHAC respiratory virus report. 

The figures are murky, however, given limited COVID testing in the community and varying trends between regions. Roughly half of Canada's wastewater sites are showing no recent change in SARS-CoV-2 trends, for instance, while a quarter are showing drops and the other quarter are showing increases.

Still, ever-evolving versions of the virus could lead to increased infections in the months ahead, much like the pattern in previous Omicron years, said Dr. Zain Chagla, an infectious diseases specialist with McMaster University in Hamilton, Ont.