Every day, more and more Canadians are vaccinated against COVID-19. It has often been suggested that as this trend continues, we will soon reach a point where so many people are invulnerable to the virus that it will largely die out due to a lack of hosts to infect — offering indirect protection to the entire population, even those who are not immunized. The goal of "herd immunity" is often described as just around the corner, signalling a return to pre-pandemic normality. But will we actually get there? Probably not, say epidemiologists and mathematicians who have been studying the latest numbers closely. At least, not any time soon. These experts highlight several major barriers that stand in the way: the arrival of new COVID-19 variants, vaccine hesitancy among adults and the fact that millions of children can't be vaccinated at all. Even in a best-case scenario, this makes the herd-immunity threshold a high bar. Practically speaking, it could be out of reach. Workers are vaccinated at a clinic in Montreal. The exact percentage of the population that needs to be fully vaccinated or recovered from COVID-19 in order to reach herd immunity is uncertain and can change. Recent estimates put the threshold as high as 80 or close to 90 per cent.(Paul Chiasson/The Canadian Press) But that's no reason to despair. It's not herd immunity or bust. Unlike politicians, epidemiologists don't obsess over a precise number, which can be hard to pin down. In practice, they say, the closer we get to that threshold — whatever it actually is — the better. And as more people are immunized, life can begin returning to something that feels much more normal. Why herd immunity is such a high bar Ashleigh Tuite, an epidemiologist and mathematical modeller with the University of Toronto's Dalla Lana School of Public Health, has run the calculations over and over — and to her, the results look pretty clear. "I think it's likely that we won't achieve herd immunity," she said. "But I don't think that's a bad thing." The reason it looks out of reach, to Tuite, has to do with cold, hard math. WATCH | Why it's difficult to pinpoint when herd immunity will be reached: The herd-immunity threshold is calculated using an equation with two main variables: the transmissibility of a virus and the effectiveness of our immune response — which, at a population level, comes from a combination of vaccination and natural infection. The exact percentage of the population that needs to be fully vaccinated or recovered from COVID-19 in order to reach herd immunity is uncertain and can change. Earlier in the pandemic, the threshold was estimated at about 70 per cent. But more recent estimates range as high as 80 or even close to 90 per cent. One of the biggest reasons for the increase is that new variants are estimated to be 50 to 60 per cent more transmissible than the original "wild type" strain of the virus. That alone significantly raises the herd-immunity threshold from what it used to be. Further complicating matters is the fact that neither vaccination nor natural infection confers perfect immunity. Questions remain about the potential for a vaccinated person to carry small amounts of the virus without having any symptoms but still being able to transmit the virus to non-immunized people. Similarly, questions remain about how long immunity from natural infection will last and how it will stand up to new variants. Millions of kids left out of vaccinations Another big challenge is the fact that vaccines are only approved for Canadians aged 12 and up. Health Canada just approved an application from Pfizer-BioNTech for use of its vaccine in children as young as 12. (Previously it had only been approved for those aged 16 and up.) Still, that leaves 4.8 million kids — or 13 per cent of the population — ineligible for any vaccine. The Pfizer-BioNTech vaccine may soon be approved for kids as young as 12 in Canada, but that would still leave 4.8 million children ineligible for any type of vaccination against COVID-19.(Evan Mitsui/CBC) That throws a huge wrench in any plans to reach herd immunity, says Caroline Colijn, a mathematics professor at Simon Fraser University in Vancouver and the Canada 150 Research Chair in Mathematics for Evolution, Infection and Public Health. "We need to be vaccinating kids as soon as we can," she said. Recent modelling Colijn conducted shows an enormous reduction in viral spread in a scenario where kids as young as 10 can be vaccinated. In fact, if 70 per cent of those aged 10 to 19 were fully vaccinated, along with a higher proportion of adults, her modelling suggests we could get close to herd immunity. WATCH | Pfizer says vaccine safe, effective in kids as young as 12: "Whether you're exactly bang on that threshold or not is not the interesting point. The point is that vaccinating that age group is really, really important," Colijn said. She cautions, however, that this modelling rests on several "optimistic" assumptions. The scenarios Colijn examined assume that natural immunity protects completely against reinfection and does not wane, that new variants will not reduce the effectiveness of vaccines and that significant proportions of the adult population will choose to get vaccinated. Vaccine hesitancy among adults Without the ability to vaccinate children, close to 100 per cent of the adult population would need to be vaccinated in order to reach herd immunity, says Raywat Deonandan, an epidemiologist at the University of Ottawa. That's impossible, he says, given the significant proportion of Canadians who are vaccine-hesitant. University of Ottawa epidemiologist Raywat Deonandan says Canada can achieve the same benefits of herd immunity by maintaining some degree of public health measures along with growing levels of vaccination.(Evan Mitsui/CBC) "Vaccine hesitancy is a barrier," Deonandan said. "It's not insurmountable, but it is going to be a barrier." Recent polling has shown that Canadians have warmed to vaccines, with a growing proportion saying they'll get vaccinated as soon as they have the chance. But about 10 to 15 per cent have consistently said they will not accept a vaccine at all. On top of that, there are people on the fence. It's the behaviour of this in-between group that may ultimately determine how close we get to herd immunity — which is why a variety of efforts are underway to encourage Canadians to get their shots. In the United States, vaccine hesitancy is increasingly seen as a major and perhaps permanent barrier to herd immunity. The vaccine rollout south of the border got off to a fast start but has been slowing as time goes on and the supply of doses starts to outpace demand. Forecasting the near future What does the future hold for Canada? That's something Paul Minshull studies closely. Minshull is the founder of Scarsin Corporation, an enterprise forecasting firm based in Markham, Ont., that has turned its detailed modelling software to the task of predicting where COVID-19 will go next. Scarsin's models are constantly updated with the latest data and combined with realistic assumptions about the things we don't know for sure — such as the true effectiveness of vaccines against transmission and the actual level of vaccine hesitancy we'll see among Canadians. Paul Minshull is founder of Scarsin Corporation, an enterprise forecasting firm in Ontario that has turned its detailed modelling software to the task of predicting where COVID-19 will go next.(Evan Mitsui/CBC) The modelling boils everything down into a simple bottom line: the number of Canadians who are expected to be genuinely protected from COVID-19 through effective vaccination or natural infection. Come October — when everyone who is eligible for vaccination is expected to have had a full complement of doses made available to them — Scarsin's latest modelling forecasts 25.2 million Canadians will be vaccinated, almost all of them with full doses. That's 66 per cent of the population. But the model also makes some downward adjustments to account for the fact that vaccines aren't perfect and that vaccinated individuals might asymptomatically transmit the virus. It also makes some upward adjustments by adding in an estimate for people who are unvaccinated but have "recovered immunity" due to natural infection. Add it all up and, according to Minshull, "the likely scenario for Canada ... is that between 58 per cent and 64 per cent of the population will have immunity this year — well short of what is needed for herd immunity." A health-care worker delivers a dose of a COVID-19 vaccine at a pop-up clinic in Toronto in April. Recent polling has shown a growing proportion of Canadians say they'll get vaccinated as soon as they have the chance. But about 10 to 15 per cent have said they will not accept a vaccine at all.(Cole Burston/The Canadian Press) These results may sound disappointing, but Scarsin's modelling also predicts what effect this level of immunity will have on the severity of COVID-19. And on that front, the future looks a lot brighter. "The good news is that things are going to improve significantly over the summer and into the fall as more and more Canadians get vaccinated," Minshull wrote in a recent analysis. Less of a finish line, more of a seesaw That's the kind of future Erin Strumpf envisions for late 2021, as well. Strumpf, an associate professor in the department of economics and the department of epidemiology, biostatistics and occupational Health at McGill University in Montreal, said she believes the end of the pandemic looks more like a drawn-out affair, rather than a hard stop. "We're going to have to live with potentially fewer people in stores and retail spaces," she said. "We're going to keep wearing masks. We're going to still continue to work from home and maybe some in the office. So it'll be more of a gradual adjustment." Erin Strumpf, a health economics expert at McGill University in Montreal, says she believes the end of the pandemic looks more like a drawn-out affair, rather than a hard stop.(CBC) Herd immunity, Strumpf said, has often been likened to a finish line: Once we cross it, the race is over. In reality, she said, the next stage in the struggle against COVID-19 is more like a seesaw: The virus will flare up at different times and in different places, and we will have to respond accordingly in order to knock it back down. But overall, it won't be nearly as severe as it's been over the past 14 months. Deonandan, with the University of Ottawa, says we can achieve the same benefits of herd immunity by maintaining some degree of public health measures along with growing levels of vaccination. "If we're willing to have mitigation tools in place for a long time, we can achieve effective herd immunity," Deonandan said. "It's just not what people think about when they think about herd immunity. They think about a free-for-all."