COVID keeps evolving, but so does our immunity. Are we now at a ‘stalemate’ with this virus?

After billions of global COVID-19 infections, millions of deaths, and countless lives upended by long-lasting health impacts, we’ve finally hit a point in this pandemic where SARS-CoV-2 isn’t the fearsome pathogen it used to be.

Once thought to kill up to 20 per cent of those infected in the early days of 2020, COVID’s destructive potential is now being throttled by widespread immunity and regularly-updated vaccines.

Even so, this ever-evolving virus is with us to stay. It still causes rolling waves of infections, much like seasonal influenza or the common cold. It’s found across the globe, in animal populations from deer to cats to mink. And it keeps mutating to better dodge our front-line immune defences and re-infect us over and over. 

We didn’t stamp it out, like many hoped. Nor did it destroy everyone’s immune systems, like some feared.

Instead, as University of Arizona immunologist Deepta Bhattacharya puts it, we’re now in a “stalemate” with SARS-CoV-2.

“It’s definitely not what it was in those awful days of early 2020. There’s no doubt about that. And so far, we haven’t really seen anything that would suggest that there’s any possibility we’d go back to that,” he added. 

“I mean, there’s almost no one left who doesn’t have some form of immunity to the virus, whether it’s through a vaccine, infection, or some combination of both.”

Most Canadians had antibodies targeting SARS-CoV-2

A new study, published today in the Canadian Medical Association Journal (CMAJ), highlights that level of immunity built up across the Canadian population.

Exposure to this virus, whether through vaccination or infection, means three-quarters of the country had detectable antibodies by March 2023, the research team found. (Those antibodies can wane and become tougher to detect over time — and don’t represent longer-lasting immune memory — which means the level of population immunity to protect people against dire outcomes might be even higher.)

The scientists analyzed more than 700,000 individual samples, said one of the study’s authors, Dr. David Buckeridge, the scientific lead for data analytics at the COVID-19 Immunity Task Force, and a professor in the School of Population and Global Health at McGill University in Montreal. The findings showed early vaccination efforts fuelled an immunity boost, though the major jump followed the arrival of the highly-contagious Omicron variant.

“These antibodies are a pretty good window on immunity,” Buckeridge said. “It shows us what we’ve been through and how the population responded, both in terms of how quickly we were vaccinated, the impact that had, but then also when new variants arrived, how that managed to escape the vaccination and cause that rapid rise in infection.”

The new Canada-wide study follows earlier research in B.C. which also suggested rising levels of antibodies in that province by the summer of 2022.

Researcher Dr. Danuta Skowronski, the epidemiology lead for influenza and emerging respiratory pathogens at the B.C. Centre for Disease Control, said the CMAJ paper further reinforces those provincial findings.

“This virus is established in the population, it’s not going away, we’re not going to drive it back into nature — we are long past that hope,” she said. “What we are aiming for now is preventing severe outcomes from infection.”

“Hybrid immunity” linked to prior vaccination and infection is a robust shield, even more so than just one type of exposure, Skowronski said. “And by that I mean stronger, longer-lasting, and more cross-protective against different variants.”

One Canadian study of health-care workers in Quebec published in The Lancet Infectious Diseases in January, for instance, found two doses of an mRNA vaccine and a previous Omicron infection offered substantial protection against future infection from existing Omicron subvariants.

That’s good news at a population level, but Skowrowski warned that seniors remain at the highest risk of severe outcomes even if they’ve been vaccinated — since many lack the potent protection provided by hybrid immunity, making first-time infections a gamble.

“Based on what we know now, the risk of severe outcomes from COVID-19 is very low for most children and young adults … where the concern still remains quite acute is for older adults,” she said.

Tailored booster shots in the works

The population-wide immune boost since the start of the pandemic comes as drug makers are tailoring the next round of booster shots to better match currently circulating variants.

Pfizer, Moderna, and Novavax have all announced their vaccines are being adapted to target XBB.1.5, an offshoot of Omicron that’s evolved to evade the first line of immune defences. 

It’s a cousin of EG.5, another similar Omicron subvariant that’s rising fast in more than 50 countries — making up roughly 36 per cent of cases here in Canada between July 30 and Aug. 5, according to data from the Public Health Agency of Canada.

While scientists say the next generation of vaccines are fairly well-matched to this evolving virus, Omicron’s savvy mutations mean re-infections will remain a reality even if people get a booster shot. 

That’s because as population immunity built up, SARS-CoV-2 either had to “adapt or perish,” said Dr. Peter Jüni, a professor of medicine and clinical trials at the University of Oxford who previously led the now-defunct Ontario COVID-19 Science Advisory Table.

“The virus upped its game by predominantly becoming more transmissible,” he said.

Over time, SARS-CoV-2 developed mutations helping it slip past early immune defences, allowing it to infect people even if they’d had an infection or vaccine dose relatively recently — though the shots do still provide protection against those infections developing into serious, life-threatening illness.

Amid that push-and-pull between the virus and our immune systems, recent waves of infections were better controlled by widespread hybrid immunity, Jüni added, which decreased pressure on health-care systems and led to fewer deaths. He’s expecting similar blunting this fall as well. (Federal data shows the weekly death toll from COVID in Canada has been dropping since the start of 2023, with a handful of deaths reported in recent weeks — down from highs in the hundreds earlier in the pandemic.)

“We also saw that the infection fatality ratio, that’s basically the number of deaths per estimated numbers of infections out there, continued to decrease, and went in the direction of what we’re accustomed to with influenza,” Jüni said.

“And this is because of this coexistence of the virus with us as immune individuals.”

COVID ‘becoming part of our reality’

Longer-term, the scientists who spoke to CBC News said they’re increasingly hopeful that SARS-CoV-2 will stay on its current evolutionary path, reducing the chance of a new variant that could dramatically spike infections or render vaccines ineffective.

But since front-line immunity can wane over time, while someone’s personal risk level may shift due to aging, pregnancy, or the development of other health issues, booster shots will still play a role in keeping COVID at bay, Bhattacharya said. 

“Going back to where we were in 2020 via waning is extremely unlikely,” he added. “So it’s really just a matter of trying to improve the degree of protection by shoring up those antibodies and reducing the chances that the virus can infect you in the first place — and making it harder for the virus to spread rapidly once it’s in you.”

Older adults in particular should be prioritized for booster shots this fall, Skowronski stressed.

Yet, for most healthy individuals, SARS-CoV-2 may be joining the ranks of other common coronaviruses that are known for causing a common cold, Bhattacharya said. In other words, it could remain a nasty, unavoidable nuisance at best, and an ongoing health threat for certain populations at its worst.

“Many of us will have it without even noticing, just having a bit of a sniffle or no symptoms at all, it may well be,” echoed Jüni. “And so it’s becoming part of our reality.”

Skowronski, however, said given the risks still faced by certain populations — and the possibility of a new variant one day emerging — COVID shouldn’t be characterized as a mere cold. Even decades down the line, when most people’s first exposures to this virus would happen early in life, she said it may remain a serious health threat to seniors, much like influenza is now.

“COVID-19’s story has not yet been fully written,” she said. “We’re still reading the pages as we go.”