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Albert Health Minister Tyler Shandro and Alberta Premier Jason Kenney unveil an opening sign after speaking about the Open for Summer Plan and next steps in the COVID-19 vaccine rollout, in Edmonton, on June 18, 2021.JASON FRANSON/The Canadian Press

As Alberta moves to lift virtually all of its public-health measures related to COVID-19, Premier Jason Kenney has responded to concerns about the speed of his reopening plan by pointing to something not included in vaccine statistics: immunity among people who have recovered from the virus.

The province plans to end restrictions on Canada Day, two weeks after reaching the threshold of 70 per cent of people 12 and older with at least one dose of a vaccine. Alberta has among the lowest vaccination rates in the country and, when looking across the entire population, the rate is even lower – about 60 per cent.

But Mr. Kenney argues that focusing on those statistics misses a substantial portion of the population who have protection even without a vaccine because they have had the virus. He has complained that news coverage about the pandemic has neglected to focus on that fact.

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Alberta’s Chief Medical Officer of Health, Deena Hinshaw, has said the province used serology testing earlier in the pandemic to estimate how many have had the virus. The tests looked for antibodies in samples from patients who had unrelated blood tests.

Dr. Hinshaw said health officials then used those earlier numbers to estimate how much testing may have undercounted actual infections by now, concluding that 12 per cent to 14 per cent of the population may have immunity “over and above” those who have been vaccinated.

Dr. Hinshaw said earlier this month that those calculations included estimates of vaccinations to ensure people weren’t “double counted.”

Both the Premier and Dr. Hinshaw have said those estimates of postinfection immunity were factored into Alberta’s reopening plan. The issue hasn’t received the same attention elsewhere in Canada as other governments implement their own plans that also focus on vaccination rates.

Experts are questioning those calculations and the decision to rely on them to guide the province’s reopening plan. They argue that it’s not clear how long immunity lasts after someone has been infected, whether that immunity could be weaker against newer variants, and how many of them have also been vaccinated. Federal and provincial health officials have also recommended that people who have had COVID-19 still get vaccinated.

“It’s a real thing for sure – the number of people who have actually had COVID-19 adds to your measure of herd immunity,” said Jim Kellner, an infectious-disease expert and pediatrician at Alberta Children’s Hospital in Calgary.

“The question is: If you had COVID-19, do you have the same level of protection and the same ability to prevent transmission as somebody who hasn’t?”

Dr. Kellner said researchers still don’t know the precise level of protection that a COVID-19 infection provides, how long it lasts, or if people who have mild cases – which may explain why they weren’t tested in the first place – have the same level of protection as someone with more noticeable symptoms.

Dr. Kellner is also skeptical that the number of people who have been infected with COVID-19 but who haven’t been vaccinated is as high as the province is estimating. He said other serological testing hasn’t found such high rates of COVID-19 infection in the general population, and whatever the number, he said it would be difficult to estimate how many of them will also get vaccinated.

“We’re encouraging and it’s recommended that people who’ve had COVID get vaccinated, and you could argue that people who have had COVID may be more likely to get vaccinated, having had the experience,” he said.

Stephanie Smith, an infectious-diseases expert and associate professor at the University of Alberta, said there is so much uncertainty around postinfection immunity – how much of it exists and what protection it offers – that it doesn’t make sense to put much weight on it when shaping policy.

Dr. Smith said there’s no question that getting infected with COVID-19 leaves someone with a strong immune response. Reinfections continue to be rare.

However, she said there’s not enough evidence yet to know whether an infection from the original, or “wild,” form of COVID-19 leaves someone with the same protection against variants that have now become dominant.

“I think that we should just be looking at vaccination rates, because it’s a hard outcome, whereas trying to estimate how many people have natural immunity is a little bit more difficult,” Dr. Smith said.

“It’s a bonus if we have people who have immunity and have chosen not to get vaccinated, but I don’t think that it’s something that we can use for our targets.”

University of Calgary epidemiologist Craig Jenne said some viruses leave patients with better immunity than others. For example, he said someone who’s had a measles infection has a much stronger and longer-lasting immunity than someone with the flu, because influenza viruses change rapidly. COVID-19 poses the same problem.

Dr. Jenne said research elsewhere in the world suggests more contagious variants are better able to infect people who previously had the original form of the virus.

“Although you were sick and recovered, the variants now are different enough that they’re able to break through that level of natural immunity,” he said.

“I think we have to be careful about overvaluing natural immunity for this particular virus. We cannot dismiss the fact there is some degree of protection offered, but it is not nearly as robust as what people believe it to be.”

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