The majority of Canadians have caught COVID-19 since Omicron and its highly contagious subvariants triggered enormous waves of infection this year — with new estimates suggesting almost two-thirds of the population have had it.
Researchers in British Columbia analyzed close to 14,000 blood samples in the Lower Mainland throughout the pandemic to track antibody levels in the general population, and found a massive shift in the number of infections in the past few months.
The B.C. Centre for Disease Control and Prevention (BCCDC) and University of British Columbia team previously found almost half previously had COVID as of April, but their new data suggests that number continued to skyrocket into the spring and summer.
The findings were published in a preprint study co-authored by B.C. Provincial Health Officer Dr. Bonnie Henry this week, which has not yet been peer reviewed, that found more than 60 per cent of the overall population had antibodies from prior infection by August.
“It really shows us the change that we saw, particularly when Omicron came … what we’re seeing now is that people may have an infection on top of being vaccinated and that has led to a level of potential immunity,” Henry told CBC News.
“We don’t know how much antibody you actually have to have to be protected, but it does give us a good sense that we have a very high level of population protection against this virus now.”
The data is also broken down by age groups and found the highest level of infections by far were in Canadians under 19, with at least 70 to 80 per cent of youth now showing evidence of prior infection — compared to about two-thirds as of April.
But adults are also seeing an increase in COVID, with at least 60 to 70 per cent of those aged 20 to 59 years old now showing evidence of previous infection, and about 40 per cent of Canadians over 60 — up from just 15 per cent as of March.
“What we observed is that children currently appear to be the most infected and least vaccinated, whereas elderly adults remain the most vaccinated and least infected,” said Dr. Danuta Skowronski, epidemiology lead at the BCCDC and co-author of the preprint.
“The main takeaway for me and my colleagues from this most recent update is that the elderly are particularly now reliant on vaccine-induced protection alone as we enter the fall of 2022, and so they should remain the priority for additional booster doses.”
The data shows huge surges of infections after Omicron landed in Canada in late 2021 and raises new questions about what that level of population immunity means for the future as updated vaccines begin to roll out across the country targeting the highly contagious strain.
“We’re in a completely different place than we were even a year ago, but particularly from the beginning of the pandemic,” Henry said.
“So what that tells me is that there are very few people now who are at extreme risk like they were early on when we didn’t have any immunity at all in anybody — and we need to pay attention to that.”
‘Good news and bad news’
The research also coincides with national data from the federal government’s COVID-19 Immunity Task Force that suggests close to 60 per cent of Canadians from coast to coast had been previously infected up to July — a massive increase over the past year.
The task force also released new surveillance data Monday from Canada Blood Services that showed an increase in COVID infections among more than 30,000 Canadians who donated blood in July — jumping from 50 to 54 per cent across all age groups.
But in younger age groups that rate was much higher, particularly among those 17 to 24 who had a rate of prior infection at more than 71 per cent.
“It’s good news and bad news,” said Dr. David Naylor, who led the federal inquiry into Canada’s national response to the 2003 SARS epidemic and now co-chairs the federal government’s COVID-19 Immunity Task Force.
“The upside is that there is a lot of background immunity arising from vaccines and past infection,” he said, noting that the combined “hybrid immunity” should help limit the impact of COVID as classes move indoors and in-person work returns.
“That said, the growth in hybrid immunity is a decidedly mixed blessing. It signals substantial ongoing spread of [the virus]. That in turn means more people with post-COVID symptoms of variable severity and duration.”
Naylor said that while most infections are mild, many people can be “laid up for days” recovering, and COVID continues to play a role in a substantial number of deaths among the most vulnerable — a toll that is likely to rise during the move into fall and winter.
“We obviously don’t want anyone to get COVID-19, and the whole goal is to reduce the risk of infection and reduce the frequency of infections,” said Dr. Isaac Bogoch, an infectious diseases physician at Toronto General Hospital.
“But in the same breath, we can’t ignore the protective benefit that recovery from infection provides people, especially when it’s in the context of vaccination as well.”
Bogoch said that underlying immunity in the population not only provides individual-level protection, but also given that so many people have been infected and recovered from COVID — it also adds to community level protection as well.
“We now have this baseline level of immunity in the population that protects everybody, it protects those people who don’t mount a good response to the vaccine as well,” Henry said. “So people who are really susceptible to severe illness are much, much reduced now.”
High level of infection, vaccination a ‘silver lining’
How this evolving population immunity holds up against future possible surges of COVID remains to be seen, but the fact remains that there are many vulnerable groups that are still at increased risk and should be prioritized for additional vaccinations.
“We know who’s more vulnerable to COVID. Obviously, we know it’s older individuals, we know it’s people with underlying medical conditions that put them at greater risk,” Bogoch said, noting that lower income and racialized communities are also disproportionately affected.
“So many people were infected over the last six months, when you add that to high levels of vaccination, at least with first and second doses — there’s room for improvement on booster doses … but we do have some significant community level protection.”
Skowronski said the findings have important implications on where future boosters are prioritized — with older Canadians, immunocompromised and other vulnerable groups being the prime concern.
“Superimposing that Omicron infection boost on top of an already highly vaccinated population is contributing to more robust hybrid immunity and that is indeed a silver lining. It puts us in a much better position going into potential further waves,” she said.
“And this is why I say the sense of urgency related to additional booster doses can be eased for the majority of the population that has accumulated now that combination of vaccine and infection-induced exposures contributing to hybrid immunity.”
Bogoch says he’s “cautiously optimistic” that updated “bivalent” vaccines, which target both the original virus and the first Omicron variant, BA.1, will provide some long-lasting protection against further spread of the virus — but their impact has yet to be seen in the real world.
“Having said that, I’d rather have a vaccine that is more closely tailored to the circulating virus now, than not, but I don’t know what the durability of protection will be,” he said.
“And quite frankly, even the old vaccine has very durable protection against severe infection, hospitalization and death. It does, it stands up over time, it’s fantastic. And hopefully we’ll retain that protection against severe infection and regain some durable protection against infection.”
Henry said that vaccination has made a “massive difference” in Canada, and despite the fact that we’ve seen high levels of COVID infection and transmission — the situation has changed dramatically.
“I think it’s a hard concept, because we spent a lot of time telling people getting COVID is really, really bad,” she said, adding that is especially true for immunocompromised Canadians and other vulnerable groups.
“Now, because so many people have been vaccinated and we have this hybrid immunity in so many people, it really reduces the risk to everybody around us.”