Got a summer cold? It could be COVID. Here's why this virus isn't seasonal quite yet

This story is part of CBC Health's Second Opinion, a weekly analysis of health and medical science news emailed to subscribers on Saturday mornings. If you haven't subscribed yet, you can do that by clicking here.

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.

New variants circulating

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. 

WATCH | Seniors remain at higher risk of serious COVID infections: 

New COVID-19 data warns of risks to seniors not yet infected

7 months ago

Duration 1:47

New Canadian data shows that more than four in 10 elderly adults may have avoided a COVID-19 infection so far. But they remain at the highest risk for hospitalization and death and researchers say keeping up-to-date with vaccinations is still the best way to reduce that risk.

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.

"We could start seeing early trickles of a wave toward the late summer," he added.

U.S. medical experts are also warning of similar trends there, albeit with smaller expected infection counts than prior summer waves.

COVID still causing hospitalizations, deaths

It's another reminder that COVID is here to stay. But with overall case counts and death rates dropping — thanks in part to higher levels of immunity across the population — it's easier to brush the virus aside.

The threat has certainly lessened since the early days of the pandemic, Chagla said. 

Yet this virus keeps hospitalizing vulnerable people, even through the spring and summer months. Some older and immunocompromised individuals are dying, Chagla said, and people with established immunity through vaccination or prior infection do occasionally develop severe disease.

As late as 2023, one U.S. study found COVID remained more deadly than influenza. This virus continues to kill in Canada: 23 people here died of COVID in just one week in May, according to the latest PHAC data.

Given how contagious the virus is and how fast immunity against infection fades, Adalja stressed that higher-risk groups — older adults, and those with other risk factors such as being overweight or pregnant — should continue approaching COVID differently than someone who's at an average risk.

"Those people, it's very important for them to be up-to-date on their vaccinations," he said. 

Vaccination remains a challenge

Public apathy toward the pathogen could complicate that.

Vaccine uptake has dropped off over the years, with fewer than two in 10 Canadians getting the most recent updated shots. And although age has always been a risk factor for severe COVID, only 53 per cent of adults aged 80 and up had their latest round, according to PHAC.

"That's the highest vulnerable group," said Chagla, "and you can't even convince more than half of them to be vaccinated."

For those who opt for additional shot, there's good news: COVID vaccines continue to be updated to better match circulating strains. This week, U.S. officials approved fall shots based on the JN.1 lineage, and Canada tends to be in lockstep with those decisions south of the border. 

WATCH | How wastewater monitoring helps track COVID:

How does wastewater help with surveillance of COVID-19?

3 years ago

Duration 4:19

Ottawa has been testing its wastewater to get a better understanding of how much of the coronavirus is in the city's sewage. The project's co-lead investigator explains how it helps with surveilling COVID-19.

But physician-epidemiologist Dr. Nitin Mohan, an assistant professor at Western University in London, Ont., said the lag between the emergence of new variants and the approval of matched vaccines remains a constant challenge. That means the world is always a step behind the evolution of SARS-CoV-2, allowing it to continually circulate and reinfect people more often than many older viruses.

"Hopefully we get to the point one day where we have vaccinations that prevent transmission," Mohan said. "I think that will be sort of the real game changer."

Until that day comes, he said it's anyone's guess how long it will take for SARS-CoV-2 to become more seasonal and predictable. 

"I probably have a better guess of when the Leafs will win the Stanley Cup at this point."