Why “ideal” herd immunity to COVID-19 is likely to be impossible — but trying to get there is still important

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In the early stages of the pandemic, herd immunity was all about defeating COVID-19 when vaccines were still a hopeful idea and variations of concern had not yet emerged.

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Most researchers looked at the replication rate of the original virus strain and thought that the community needed to immunize 60-70% of its population in order to starve the new body to infect COVID-19. (By either vaccination or capture of the virus). That’s where the pandemic ends effectively.

But that number now seems quaint. Despite nearly three-quarters of Canada’s total population being vaccinated, cases of COVID-19 are rising again in many states, with Alberta and Saskatchewan seriously affecting the health system. We are recovering from the massive surge that threatens us.

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Canada’s national cases declined last month, but Canada’s chief public health officer, Dr. Theresa Tam, told reporters Friday that the latest model shows that the rate of decline has slowed. rice field. “It’s not surprising that we’ll see some bumps in orbit in the coming months,” Tam said.

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What we really want to do is distribute immunity to the population to minimize medical demand.

The game has changed significantly due to highly infectious variants, breakthrough infections, and reduced vaccination immunity. Many modeling experts now believe that it is no longer possible to achieve optimal levels of group immunity with COVID-19. More precisely, I believe we need to start talking about group immunity in another way.

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“There is this kind of idealized herd immunity. A sufficient number of people are immunized and the virus dies and disappears,” said Simon Fraser, Canada’s 150 research chair in evolutionary and infectious mathematics. Professor Caroline Colleen of the University said. And public health.

“COVID-19 doesn’t get that. It doesn’t get the ideal herd immunity,” Colijn said. Simply put, vaccines are not perfect and the virus continues to mutate to spread more easily.

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“But if you need to guess or predict, you’re likely to get a kind of practical herd immunity. This is enough immunity to restart the population. You can establish an almost normal life, “she said. “It may be new and normal in a sense, but it’s mostly normal social and economic activity without overwhelming the healthcare system with COVID.”

Professor Jane Heffernan of the University of York, who specializes in disease modeling and mathematical epidemiology, agreed that herd immunity in the sense of eliminating the virus is now likely off the table.

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“It’s really hard to achieve herd immunity,” she said. “We could actually do that for only one virus, smallpox.”

But she said aiming for a high level of immunity would take us to places where the virus does not control our lives.

“I think we should reduce the stress on herd immunity and increase the stress on maintaining high levels of immunity that protects people,” Hefernan said. “What we really want to do is distribute immunity to the population so as to minimize medical demand. I really think it is not realistic for us to achieve resistance.”

Professor David Earn of McMaster University, an applied mathematician specializing in epidemiology, said that herd immunity traditionally meant the level of herd immunity. Spread. From his point of view, the goal is very simple. The number of immunizations should be as high as possible.

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I hope things are pretty stable

“The higher the degree of herd immunity, the better for everyone,” he said. “But is there an ideal level of herd immunity that can actually be achieved? Unfortunately, I don’t think it is anymore.”

(Colijn, Heffernan, and Earn have all worked to model the forecasts of public health authorities and scientific advisory boards at the state and federal levels.)

Speaking on Friday, Tam said it was very difficult to specify the level of community immunity to stop the spread of the virus due to the ever-changing landscape and local factors.

“That’s not an easy question,” Tam said. “As a stadium, it is a virus with a rebirth rate of 5-6, which may be a Delta variant in certain situations. In that case, at least 80 percent of total herd immunity must be achieved. This is 12 It’s not the target population over the age, but it’s 80 percent of the total population. “

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Health Canada’s website currently shows an immunization rate of 73.96 percent of Canada’s total population. Assuming that vaccinations are approved between the ages of 5 and 11, Canada could soon be very close to 80% of that, but not beyond, because we are currently in the United States. But Tam repeatedly warned that she was only giving a quote, which could change in the future.

For COVID-19, you won’t get it. Ideal herd immunity cannot be obtained.

“It’s actually very difficult to pinpoint a particular number, but we’re talking about that kind of stadium with a virus that acts like a delta virus,” she said. “So our recommendation, and certainly my message, is to really make it as high as possible.”

If there is a general consensus that herd immunity is unlikely to reach the point where COVID-19 disappears, it raises another question: how far from the point where herd immunity exists. teeth Is the virus high enough to be managed with almost no public health restrictions?

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“That’s the problem people are working on right now,” Heffernan said. “There is a lot of controversy about that, and it depends on the generation of new mutant strains of concern.”

Colijn said she is optimistic that the modeling work she did at BC is not far from the level of sustainable immunity that brings us back to a relatively normal life. However, she said she was likely to have to undergo public health measures such as mask obligations for some time, and also hopes that Canada will more fully embrace rapid testing as a ubiquitous screening tool.

“Ultimately, no matter what you’re still doing about COVID, you want to be comfortable in the long run,” she said. “And to get there, either infection or vaccination may require more immunity, but I think we’ll get there.”

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There are other innovations that may make it possible to keep the virus at a manageable level. For example, in the end, a situation could occur in which the seasonal influenza vaccine also includes the COVID-19 booster.

Meanwhile, Pfizer announced on Friday that its antiviral drug would show a highly effective treatment for COVID-19, reducing the risk of hospitalization or death by 89%. Assuming it lasts and the pill is approved in Canada, containing the effects of the virus will be a powerful new weapon.

Arn said he was generally optimistic about where we were heading. The big picture is that vaccines are still very effective in preventing serious illnesses, and once approved between the ages of 5 and 11, vaccination levels can rise significantly.

“I’m optimistic, but I’m pretty optimistic,” he said. “There are many ways things can still go wrong. Obviously, it would be terrible if there were a new variant that evaded the vaccine. It would be a lot of setbacks. And unfortunately it is possible. But I hope it doesn’t happen. If it does, I have to deal with it. In the meantime, I hope things are pretty stable. “

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