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Gisele Levesque, one of 229 residents at the CHSLD St-Antoine long term care home, is injected with the Pfizer-BioNTEch COVID-19 vaccine in Quebec City, on Dec. 14, 2020.PAT LACHANCE/Reuters

In the first five days of its COVID-19 immunization campaign this week, Canada managed to vaccinate slightly more than 9,000 people. Unfortunately, more than 33,000 new coronavirus infections were reported in the same five-day period.

If the virus and the vaccine are in a race, the virus has a massive head start. But as the pandemic marathon grinds on, the vaccine will inevitably cut into the coronavirus’s lead, and new questions will arise.

What counts as the finish line? Will Canadians be able to return to their old lives once health care workers and the elderly have been immunized? Or will they have to wait for herd immunity or even outright elimination of the virus?

“I know people want the social distancing and physical distancing to go away,” said Jane Heffernan, director of the Centre for Disease Modelling at York University. “But if [restrictions] are relaxed too early, we’re going to see these being enforced for a longer time.”

Dr. Heffernan is one of several epidemiologists and mathematicians trying to project what will happen to Canada’s raging epidemic as the vaccines roll out. Their crystal balls are cloudy because there are still so many unknowns.

Dr. Heffernan, for example, is modelling “immunity decay” to see what lies ahead if the protective effect of vaccines wears off in a few months or lasts for years.

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Other modellers, including Chris Bauch of the University of Waterloo, are looking at what could unfold if vaccine recipients are prioritized in different ways.

Back in September, before the results of any of the late-stage COVID-19 vaccine trials had been released, and before anyone knew how bad the fall wave would be, Dr. Bauch and his colleagues set out to find the most effective order in which to give out hypothetical shots.

They ran the numbers, and found that if a 90-per-cent effective vaccine were distributed beginning in mid-December, 2020, the best way to save lives would be by starting with people 60 and older, even if older adults tend to have fewer contacts than their younger counterparts and are therefore less likely to spread the virus.

The balance tipped toward vaccinating younger spreaders first if vaccines didn’t become available until July, 2021, Dr. Bauch and his co-authors explained in a paper published on a preprint server, meaning it has not yet been peer reviewed.

The first imagined scenario, it turns out, is strikingly similar to where Canada is today.

Health Canada has authorized a COVID-19 vaccine from U.S.-based Pfizer Inc. and its German collaborator BioNTech, and it is on the cusp of approving another shot by Moderna Inc. Both vaccines were around 95-per-cent efficacious against COVID-19 in clinical trials.

Assuming adults older than 60 are immunized in the early part of the blitz, Dr. Bauch and his colleagues project that COVID-19 deaths would drop by somewhere between 32 per cent and 77 per cent, depending on the speed of the rollout.

“Herd immunity is not the only consideration. Herd immunity is a means to an end,” Dr. Bauch said. “Ultimately, what we want to do with the vaccine is prevent the most harm. And initially, that might mean that we should first immunize the elderly, because the death rate is so high in that age group.”

Herd immunity is the point at which enough people in a community are immune to a virus – either through vaccination or infection – that the pathogen struggles to find susceptible hosts.

Most experts estimate that around 70 per cent of the population would need to be immune to the coronavirus for it to peter out.

In Canada’s case, that would require vaccinating about 26.6 million of the country’s 38 million residents, a target that is harder to reach than it seems because, at least at first, the vaccines won’t be authorized for children and most teens.

Remove Canada’s eight million children and teens from the equation, and the vast majority of adults would have to be willing to roll up their sleeves, something that might not be possible if a sizable chunk of the population declines the shots.

At the other end of the age spectrum, there are 1.6 million people older than 80 and 9.3 million older than 60.

For Daniel Coombs, a University of British Columbia mathematician who has been developing COVID-19 models for the BC Centre for Disease Control, trying to project how younger people will behave after their parents and grandparents have been immunized is the trickiest part of modelling the impact of vaccines.

“Once the care homes are vaccinated, the drop in the death rate is going to be massive,” he said. “One of my biggest concerns is that, at that point, the provincial authorities and just general people will take their foot off the gas and we’ll actually see cases go up in younger age groups.”

Political leaders will have to decide how much that matters. For a time, they’ll be presiding over a kind of vaccine purgatory, balancing unchecked spread of the virus among lower-risk groups against the economic and psychological toll of continued pandemic restrictions.

Fortunately, there is a way out of that limbo.

“I think the goal should be to vaccinate every Canadian who wants a vaccine,” said Irfan Dhalla, a vice-president at Toronto’s Unity Health. He’s also an internal-medicine physician who just finished a stint on the COVID-19 ward at St. Michael’s Hospital.

“If we are able to achieve that kind of coverage, we are highly likely to have herd immunity, and there will be very few or no cases of COVID-19 circulating in our community.”

The initial COVID-19 vaccinations in Canada and around the world raise questions about how people react to the shot, how pregnant women should approach it and how far away herd immunity may be. Globe health reporter Kelly Grant and science reporter Ivan Semeniuk discuss the answers.

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