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People who just received their jab against COVID-19 wait for their vaccine card to be processed in Orange Farm, South Africa, on Dec. 3, 2021. Canadian scientists are paying close attention to the mounting evidence out of South Africa and other countries that Omicron is more transmissible than the Delta variant.Jerome Delay/The Associated Press

Infectious disease experts are narrowing their focus on a pair of numbers that will likely determine the trajectory of the Omicron variant in Canada and shape the country’s public-health response.

On one hand is the rate of infection among vaccinated individuals; on the other is hospital admissions.

Put the two together and – so far – the results looks somewhat favourable for Canada because of its high vaccination rate, said Peter Juni, an epidemiologist at St. Michael’s Hospital in Toronto and the scientific director of Ontario’s COVID-19 science advisory table.

While Omicron is certainly spreading, serious cases in South Africa appear to be showing up mostly among the unvaccinated. Based on an update from South African colleagues Friday, Dr. Juni said unvaccinated individuals may account for more than 85 per cent of hospital cases there.

“That’s really important,” he said. “These are preliminary data, but what we see is that there is still a high extent of protection against hospital admissions for fully vaccinated people.”

The corollary to this observation is that vaccination against COVID-19 – as much and as soon as recommended – remains the best available defence against death and disease. That means third doses are a must for anyone who is eligible, second doses for those who need them, and first doses for children five to 11 years of age and anyone who has been holding back until now, said Dr. Juni, who noted that the Johnson & Johnson product is an option for people reluctant to take an mRNA vaccine.

“We need to optimize our immunity,” he said.

But while it will be very good news if vaccines continue to prevent severe disease from the new variant, that does not translate into a business-as-usual scenario, several experts told The Globe and Mail. This is because of mounting evidence that Omicron is more transmissible than the Delta variant, which is currently dominant, and potentially better able to spread among the vaccinated population.

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If Omicron is even modestly more transmissible than Delta, it can be expected to eventually take over, just as other variants have before it. Because of its timing, it may be further aided by waning immunity in the population and seasonal effects caused by people spending more time together indoors.

The outcome of this is a likely increase in the current rate of infections, probably leading to more hospitalizations even without an increase in disease severity. What concerns health experts is how fast this might take place and how much strain that may put on Canadian hospitals.

As a comparison, both the Alpha and Delta variants displaced their predecessors in about four to five months when they arrived in Canada. Each was estimated to be about 50 per cent more transmissible than the variant that preceded it.

But early estimates for Omicron suggest a transmissibility that ranges from 66 per cent to more than 500 per cent higher than Delta’s – a stunning increase.

“Some of these estimates are almost implausible because they’re so crazy high,” said Troy Day, a computational biologist at Queen’s University in Kingston who also sits on Ontario’s science table.

Dr. Day said that, for now, the estimates need to be taken with a big grain of salt. This is because Omicron, like all SARS-CoV-2 variants, is identified through genomic sequencing. And because sequencing efforts are prioritizing places where the variant is thought to be present, the variant’s prevalence is likely to be exaggerated in the data.

Nevertheless, he said, while the transmission rate for Omicron has yet to be accurately quantified, the accumulating evidence makes it hard to explain how it has grown so rapidly without at least some improved ability to spread.

Canadians should therefore be prepared for a variant that takes off much more quickly than they have seen before, once it becomes established through community spread.

“What we’re talking about is a matter of weeks,” Dr. Juni said.

Caroline Colijn, an applied mathematician and disease modeller at Simon Fraser University in Burnaby, B.C., said one of the challenges in predicting precisely what to expect from Omicron is teasing apart its transmissibility from its apparent ability to spread among the vaccinated.

Antibody studies may shed light on this in the next week or two, Dr. Colijn added, but broad population-wide data about vaccine effectiveness and Omicron are more likely to be one to three months away.

Meanwhile, individual outbreaks of the variant are already demonstrating that immune escape is a possibility. This was reinforced Friday by reports of at least 50 people in Oslo, Norway, infected with the variant because of a superspreader event at a company Christmas party that only vaccinated employees were allowed to attend.

An analysis posted online earlier in the week by researchers at the University of South Africa and McMaster University in Hamilton, Ont., also appeared to confirm a higher risk of reinfection associated with Omicron. While this is not a direct measure of vaccine effectiveness, the result is in keeping with the idea that Omicron may be better able to dodge the body’s immune response based on the number of genetic changes it exhibits.

Another confounding factor is the possibility that many vaccinated individuals may not know when they have COVID-19, which would make Omicron harder to track and assess in Canada.

Catherine Hankins, a professor of public and population health at McGill University who co-chairs Canada’s Immunity Task Force, said she and other task force members are investigating the feasibility of surveillance testing to shed light on vaccine effectiveness among people who do not report symptoms.

“We are looking at how we can pull together a picture that would give us a better understanding of what’s happening across the country” she said.

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