Dr. Kashif Pirzada, an emergency physician and faculty member at the University of Toronto, says recent illness patterns are unlike anything he has seen in his 17 years practicing medicine.
“We noticed something was strange towards the end of the summer,” said Pirzada, who also co-chairs the Canadian COVID Society, which educates Canadians on the health impacts of COVID.
Pneumonia was “surprisingly infecting a lot of children, which was not an issue before [the pandemic],” he says.
The year before, in 2023, group A strep was unusually pervasive, making headlines across the country. And in 2022, Canada experienced its largest-ever wave of respiratory syncytial virus, commonly known as RSV.
Pirzada attributes these infection patterns to SARS-CoV-2, the virus that causes COVID. In his observation, waves of unusually severe infections follow surges in COVID by about a month or two.
“We see the aftermath a couple months later in terms of strange infections. It seems like there’s a correlation,” he said.
The efforts to explain COVID’s role on changing infection patterns have coalesced around two schools of thought.
One theory, known as “immunity theft,” is gaining traction in the scientific community. It says COVID is weakening our immune systems, leaving us vulnerable to other illnesses.
The other theory, “immunity debt,” says a lack of exposure to common illnesses throughout 2020 and 2021 — due to lockdowns and mask mandates — left people more susceptible to getting sick.
‘Immunity theft’
There is broad consensus that infection patterns have changed.
Hundreds of thousands of families have been navigating a near-constant cycle of household illness since the pandemic, according to new research.
Jessica, an Alberta-based mother of a toddler who participated in that research, described her family’s experience as follows:
“It was like, every two weeks, [my daughter] was getting sick. And then we would get sick right after she got sick,” Jessica said, referring to herself and her husband. “And she got better within a few days, and then we’d be sick for like, two to three weeks, we’d still be dealing with the last virus. And once we’re feeling better, we’d get hit with the next one.”
Several recent studies point to the “immunity theft” hypothesis.
A December 2024 study of nearly 900,000 Ontarians compared COVID patients with influenza and RSV patients. COVID patients had significantly higher odds of getting sick with a bacterial infection in the weeks after recovering from the virus.
Research from November 2024 followed people with active COVID infections for one week. It found their T cells — which are critical for fighting off infections — aged rapidly, becoming less effective and possibly setting the stage for other near-term infections. Researchers concluded these observations were unique to COVID.
While cautioning that it is still early, several Canadian researchers are also coming down on the “immunity theft” side of the debate.
Dawn Bowdish, Canada Research Chair in Aging & Immunity at McMaster University, says they see immune changes following COVID infections in her lab. But she cautions against singling COVID out as uniquely disruptive.
“In our own work do we see that ‘COVID changes your immune system?’ Yes. But so does absolutely every other thing you’ve ever been exposed to,” she said. “Infections are never good for you.”
“Virtually every viral respiratory infection has this period where the immune responses needed to deal with the virus leave you compromised to bacterial infections,” she added.
Samira Jeimy, program director of Clinical Immunology and Allergy at Western University, says COVID’s disruptive effects on the immune system are probably driving recent illness surges.
“Other viruses cause immune dysregulation,” Jeimy said. “I don’t know why we’re in such denial that COVID can do it as well.”
“There’s still a pervasive belief that all of this is because of an ‘immunity [debt],’ which is hard to believe,” she said..
Raywat Deonandan, a University of Ottawa epidemiologist, said he is also “quite open” to the immunity theft hypothesis.
“We’re seeing rises in respiratory infections of all kinds,” he said. “And there’s probably something behind that.”
‘Immunity debt’
Other researchers say the “immunity debt” hypothesis still has explanatory power.
“I don’t know that one virus affects how we react to other viruses in the long run,” said Marek Smieja, an infectious disease physician at McMaster University.
He said last year’s spike in mycoplasma pneumonia, an under-researched bacterial infection, could be explained by immunity debt.
In 2020 and 2021, North America was likely due for a mycoplasma wave. But because of pandemic restrictions, that wave never came — potentially leading to a “double-cohort” of unexposed children.
But outside of edge-case infections such as mycoplasma, the immunity debt theory is losing favour with many experts.
Colin Furness, an epidemiologist and professor of information sciences at the University of Toronto, says immunity debt made sense in 2022, when the first post-lockdown surges hit.
But extending that logic to explain illness surges in 2025 “is nonsense,” he says.
Still, he stresses that immunity theft is far from settled science.
“Everyone’s getting COVID all the time, and we’re not doing any kind of surveillance,” he said. “We’re making it harder and harder to test these questions.”
‘Close our eyes’
Sabina Vohra-Miller, a PhD candidate at the Dalla Lana School of Public Health at the University of Toronto, is more forceful. She says the continued embrace of the immunity debt theory is actively undermining public health efforts.
That notion, she said, has led many to believe getting sick might be a natural way to strengthen one’s immunity, “which is not true.”
She said the popularization of immunity debt has contributed to widespread distrust in public health authorities and undermined support for protective measures implemented early in the pandemic.
Pirzada, the emergency physician, says practical policy responses could mitigate the continued toll of respiratory illnesses on population health.
He recommends regulating indoor air quality, especially in schools, increasing vaccine uptake, “which is far too low,” and educating physicians on the growing science around COVID-related immune disruption.
Epidemiologist Deonandan agrees, adding that paid sick days would also help suppress the spread of illnesses.
“As much as we’d like it to be, life still isn’t back to normal,” said Jeimy, the clinical immunologist. “As a society, we’ve taken an approach of, ‘if we close our eyes and pretend it doesn’t exist, it doesn’t exist.’”
“We would like so desperately for everything to be back to normal, that I think we’re doing some actual damage,” she said.
Jeimy questions whether the public — or policymakers — are truly prepared to confront the long-term implications of the virus. “I don’t think our society is ready for it.”
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