Why ‘ideal’ herd immunity for COVID-19 is likely impossible — but attempting to get there still matters
It’s much more practical — and possible — to aim for a level of immunity where people can lead normal lives without the interference of the virus, experts say
Back in the early stages of the pandemic, when vaccines were still just a hopeful idea and variants of concern had yet to make an appearance, herd immunity was all the talk when it came to beating back COVID-19.
Most researchers, looking at the reproduction rate of the original virus strain, figured a community would need to see 60 to 70 per cent of its population immunized (either through vaccination or catching the virus) to starve COVID-19 of new bodies to infect and effectively end the pandemic there.
But that figure seems quaint now. Despite nearly three quarters of Canada’s total population being vaccinated, COVID-19 cases are on the rise again in many provinces, and Alberta and Saskatchewan are only just recovering from massive spikes that seriously threatened their healthcare systems.
Although cases nationally in Canada have dropped over the last month, Dr. Theresa Tam, Canada’s chief public health officer, told reporters on Friday that the latest modelling shows the rate of decline slowing. “It’s not unexpected that we could see some bumps in the trajectory during the months ahead,” Tam said.
What we really want to strive for is a distribution of immunity in the population such that we’re minimizing healthcare demand
Highly transmissible variants, breakthrough infections and waning vaccination immunity have changed the game substantially. Many modelling experts now believe achieving an optimal level of herd immunity is no longer possible with COVID-19 — or, to be more precise, they believe we need to start talking about herd immunity in a different way.
“There’s this kind of idealized herd immunity that, you know, we get enough people immunized, the virus dies a death and it’s gone,” said Simon Fraser University professor Caroline Colijn, who holds a Canada 150 research chair in mathematics for evolution, infection and public health.
“We’re not going to get that for COVID-19, we won’t get that idealized herd immunity,” Colijn said. Simply put, the vaccines aren’t perfect and the virus keeps mutating to spread more easily.
“But if I had to make a guess, or make a projection, I would say we are likely to get a kind of practical herd immunity, which is enough immunization in the population that we can reopen…that we can establish a mostly normal life,” she said. “Maybe it’s a new normal in some sense, but it’s mostly normal social and economic activity without overwhelming our healthcare systems with COVID.”
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Jane Heffernan, a York University professor specializing in disease modelling and mathematical epidemiology, concurred that herd immunity in the sense of eliminating the virus is likely off the table now.