Canadians have been told that herd immunity must be achieved before some semblance of normalcy can return to their lives, but what is not yet known is what is needed to achieve that level of protection.
Herd immunity is defined as the number of people who need to be immune to a disease, most typically through vaccination, in order to prevent the spread of an infection. However, evidence is still developing around herd immunity to COVID-19 and there have been a range of estimates.
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Despite that, one thing most experts agree on is that a high percentage of the population will likely need to be immunized before a societal protection against COVID-19 can be reached.
Colin Furness, an infection control epidemiologist at the University of Toronto, said he thinks the level to achieve herd immunity is in flux. Furness said the virus’ reproductive number will play a big factor.
The reproductive number, also known as the reproductive value, R value or R naught, describes how many people one infected person will go on to infect. For example, a reproductive value of one means one infected person will infect one other. Generally speaking, a reproductive value above one means the epidemic is worsening, a value at one means it is the status quo while a value below one signals the situation is improving.
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Furness said the reproductive value of the novel coronavirus, the virus that causes COVID-19, has ranged from below one to near seven.
“It depends on local conditions so that’s a moving target. The lower the reproduction number, the less of the population you need [to vaccinate],” Furness said.
“My… calculations say 70 to 75 per cent should be fine. If you vaccinate that group, then you’re OK.”
However, Furness said his calculations assume interventions, such as masks and distancing, are still in place to keep the reproductive number low.
And while he estimates 70 to 75 per cent of the population should be immunized to try and achieve herd immunity, he would prefer the rate to be closer to 85 per cent.
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The percentage of people needed to be vaccinated in order to create herd immunity depends on the disease.
According to the World Health Organization, herd immunity against measles requires about 95 per cent of a population to be vaccinated and for polio, the threshold is about 80 per cent.
Dr. Lynora Saxinger, an infectious disease specialist at the University of Alberta, said there has been a range of estimates for herd immunity around the 60 per cent range.
“I think it’s still of value to say that once we get up to 60 per cent of the population being immune, we would not expect to see these broad, severe epidemics being able to take hold anymore.
“I just wouldn’t want to sell the idea this is a magic threshold beyond which all problems disappear and everyone is protected because that’s actually really not the case.
“That just means if it comes up again, it’s unlikely to be a huge epidemic. It’s likely to be a small flare of infection in a pocket of susceptible people.”
However, Saxinger said if it turns out the vaccine is time-limited, it will be important for people to get their booster shots to keep the level of population up.
Complicating the issue are variants, such as the ones first identified in the United Kingdom and South Africa, that appear to be more transmissible. Both have been discovered in Alberta.
Dr. Alexander Doroshenko, an associate professor in the Division of Preventative Medicine at the University of Alberta, said the greater transmissibility of these variants could increase the threshold needed to achieve herd immunity.
“That number may be higher than what we initially estimated.
“This initial estimate of herd immunity is really the starting point of the disease of what we need to achieve. Herd immunity can come from vaccinations as well as from natural disease,” Doroshenko said.
However, he said that vaccinations are a more reliable way to reach herd immunity and avoids any impacts associated with the disease.
Doroshenko said other considerations for how herd immunity is achieved include the possibility a vaccinated person could still transmit the disease, uneven transmission levels between people, and the speed at which vaccinations can be rolled out.
“None of them can determine the threshold of immunity individually. I think we should aim to vaccinate 75 to 80 per cent of the population,” he said.
“Even though I hope we may see some benefits at the lower number, I think we should aim for the higher number.”
Chief Medical Officer of Health Dr. Deena Hinshaw said Jan. 11 that studies looking at the spread of the virus show roughly 70 per cent of the population needs to be immune before herd immunity is achieved.
Hinshaw said the new variants that could change that proportion and said that, until a larger percentage of the population has access to the vaccine, “we have each other as our vaccine.”
The vaccine for COVID-19 will not be mandatory in Alberta but both Hinshaw and Premier Jason Kenney have encouraged Albertans to get inoculated.
–with files from Katie Dangerfield