Almost half of Canadians may have caught COVID

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Canada's COVID-19 immunity landscape has completely transformed since the emergence of Omicron with new estimates suggesting that almost half of the population has been infected.

Researchers in B.C. analyzed thousands of blood samples in the Lower Mainland throughout the pandemic to track antibody levels in the general population, and found a massive shift in the level of infection in the past few months.

The B.C. data, provided in advance to CBC News, found close to 40 per cent of the population had antibodies from a previous infection in March, up from around 10 per cent in October. That number is even higher in children under 10, with nearly two-thirds now showing evidence of prior infection.

"Think about that two out of three children," said Dr. Danuta Skowronski, a vaccine effectiveness expert and epidemiology lead at the British Columbia Centre for Disease Control who led the research.

"Wow, something really dramatically changed and I think we know the name of that change it's Omicron."

Close to90%vaccinated, infected or both

Skowronski said close to 60 per cent of those aged 10 to 40 also had antibodies from prior infection, and just under 50 per cent of those aged 40 to 60.There were slightly lower levels in those aged 60 to 80 possibly because they were less socially engaged and vaccinated earlier.

The data also suggests almost 90 per cent of the population has had their immune system primed against the virus in some way either through vaccination, prior infection or both.

Skowronski, whose research laid the groundwork for the decision to delay second doses in Canada, said we are in a "vastly different context" now than at the start of the pandemic, when we had basically "total population naivety" to the virus.

"It's about one in two, almost, of our population that have had evidence of infection," she said.

And while the data provides a useful snapshot to estimate the level of infection in the community, she saidit doesn't directly translate to exact population immunity levels.

"We cannot say that this is an indication of protection or being exempt from future infections, especially when new variants arise. But a primed person is immunologically a prepared person."

While the data is preliminary and has yet to be peer reviewed or published, it is in line with emerging data from other provinces, like Ontario, where official estimates now show as much as 40 per cent of the population wasinfected with COVID-19 since December alone.

"We have had millions of infections in the last few months, most of them undocumented," said Dr. David Naylor, who led the federal inquiry into the 2003 SARS epidemic and now co-chairs the federal government's COVID-19 immunity task force.

"I would not be surprised if the true number is over 10 million nationally by the end of the sixth wave."

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Canada has become a variant magnet

Canada's COVID-19 immunity landscape is one of the most complex in the world due to our high vaccination rates, widespread transmission, delaying and mixing of doses and the fact we've been a magnet for variants often with multiple circulating at once.

But does the protection we've gained from vaccines, prior infection and combined hybrid immunity mean the worst is behind us? Or could waning immunity to infection, the spread of new variants or some other pandemic curveball threaten another surge in the future?

"If there's a future variant that has infective advantages over Omicron, we can expect it to arrive and spread here," Naylor said. "That much is obvious from the last two years. SARS-CoV-2 goes pretty much everywhere at a brisk clip."

Canada's bold decision to delay and mix doses of COVID-19 vaccines led to strong protection from hospitalization and death, but the emergence of Omicron and its subvariants rendered them less protective against infection than previous virus strains.

A new Canadian preprint study, which has not yet been peer reviewed, found the effectiveness ofjust two doses of Pfizer-BioNTech, Moderna and AstraZeneca-Oxford vaccines was "excellent" against severe outcomes from COVID-19 up until September 2021.

That research follows another new study co-authored by Skowronski, and published in Clinical Infectious Diseases, that showed two doses of mRNA vaccines or AstraZeneca "gave substantial and sustained protection" against hospitalization from Delta, up to November 2021.

While vaccine effectiveness against severe outcomes has held up well, two-dose protection against Omicron infection fell dramatically in December,dropping to just 36 per cent, according to one Ontario preprint study, with boosters raising it to 61 per cent.

That significant hit to our population immunity underscored the need for boosters and left us highly susceptible to Omicron-fuelled fifth and sixth waves that have since ripped through the population.

'Complex' immunity landscapes

Tulio de Oliveira, the director of South Africa's Centre for Epidemic Response andInnovation, who led the research team that identified subvariants BA.4 and BA.5, said countries like Canada now have "very complex" immunity landscapes in the wake of Omicron.

"What that means is that what one country sees with a variant and subvariant may be slightly different from which other countries see," he told CBC News in a phone interview.

"But if population immunity is increased, especially through vaccination and boosting, that still should bode very well against not only Omicron, but potentially any new variant that may emerge."

Canada's National Advisory Committee on Immunization (NACI) strongly recommended boosting all adults and high risk children earlier this month, and de Oliveira said the most effective way to boost a population is before a new wave of infection starts.

WATCH |Canadians urged to get COVID-19 boostersto blunt 6th wave:

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"That's what countries should be concentrating on," he said. "When infection is low, it's a great idea to go into a campaign of a booster for people that have been fully vaccinated, or vaccinated and infected, because then the next wave that comes will be much less deadly."

Naylor said our prior infection immunity, combined with the current vaccine coverage, continued rollout of boosters, additional doses for kids and the "smart use" of fourth doses in eligible Canadians should provide a "fairly solid immunity wall for the summer."

"It's prudent to remember, however, that no one predicted Omicron," he said. "And so that immunity wall could still be breached."

'Running out of territory' with current vaccines

While a high rate of vaccination and previous infection in Canada doesn't guarantee the worst is behind us, it's clear that it will continue to reduce the likelihood of severe outcomes from COVID-19 for the vast majority of Canadians.

But as the virus continues to evolve into new variants that are light years away from the original strainthat the vaccines are targeted to, the push to update the formulations to target Omicron and other variants is top of mind.

"We need broader coverage from the vaccines. Much to my frustration, we're still aiming against a strain that's long, long, long gone," said Deepta Bhattacharya, an immunologist and professor at the University of Arizona.

"I understand the practical reality is these are the tools that we have on hand right now, but we're definitely running out of territory in terms of what we can expect these vaccines to do, given how different Omicron is than what the vaccines are aimed against."

Moderna announced new findings this week showing that a booster shot designed to target both the Beta variant and the original virus strain generated a better immune response against several other variants, including Omicron.

While that exact formulation will likely never see the light of day, it bodes well for the possibility of tailoring future mRNA shots to target multiple variants. But until we have updated vaccines, immunity from prior infection may work to fill the gap.

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"Given the mismatches of the vaccine with Omicron, almost certainly a prior Omicron infection with BA.1 would confer more protection against Omicron sublineages,like BA.2," Bhattacharya said.

"So the people who recovered from BA.1 are almost certainly in better shape in dealing with BA.2 than someone who just has gotten say a couple of shots of the vaccine in the past," he said.

Skowronski said the Canadian population is more immunologically prepared against COVID-19 now than ever before. And if Omicron continues to evolve new variants while close to half of us have been infected with it, there will be some "cross-protection."

"We can't predict what the subsequent variants may be. They likely will be evolutionarily related, but may not be to Omicron," she said,adding COVID-19 vaccines may have to be updated regularly by a team of international experts, much like influenza shots are.

"The question is, to what?"