When you see the daily case counts in the news at this point in the pandemic, they don't really mean the same thing as they did before — the risk to Ontarians who are susceptible to COVID-19 is higher than it looks, masked by the provincewide daily numbers.
That, as epidemiologist Dr. David Fisman explained to QP Briefing, is because of increasing immunity among Ontarians, thanks mostly to vaccination.
At this stage of the epidemic in Ontario, the vast majority of new infections are among unvaccinated people. But because vaccines aren't 100-per cent effective, there is also a smaller share occurring among those who are partially vaccinated after one dose as well as rarer "breakthrough" cases among those who've had both doses.
The upshot of that is infections are occurring among a shrinking pool of people, referred to by epidemiologists as the susceptible population.
<img class="alignleft wp-image-390774" src="https://www.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman-405x394.jpg" alt="" width="200" height="195" srcset="https://old.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman-405x394.jpg 405w, https://old.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman-843x821.jpg 843w, https://old.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman-768x748.jpg 768w, https://old.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman-1536x1495.jpg 1536w, https://old.qpbriefing.com/wp-content/uploads/2021/05/David-Fisman.jpg 2000w" sizes="(max-width: 200px) 100vw, 200px" />
And that's how epidemics die out, said Fisman. "Just like a forest fire burning through all the dry wood until there's not enough to sustain the fire, an epidemic ends because it runs out of susceptible individuals to infect."
But because Ontario's epidemic isn't over yet, and the cases that are still occurring are among a smaller group, the risk to those people — the force of infection, in epidemiological terms — remains relatively high, he said.
The chart below shows how that force of infection calculation has changed in Ontario over time. Over the course of Ontario's third wave, the province's vaccination campaign progressed significantly, from about three per cent of the population having had at least one dose to about 58 per cent having had one as of May 30. As a result, the risk to the susceptible population, compared to the entire population, diverged. The risk to vaccinated Ontarians was and is much lower — four to five times lower, based on the effectiveness of the vaccines — than to their unvaccinated neighbours.
This chart shows an estimate of the weekly COVID-19 cases per 100,000 people among the unvaccinated and otherwise susceptible population (yellow), the entire population (blue) and the vaccinated (green) based on the effectiveness of vaccines. The likely range of vaccine effectiveness in the province is between 60 and 80 per cent, according to Public Health Ontario. You can see how that range impacts the calculation by clicking on the buttons at the top left. The more effective vaccinations are, the bigger the gulf in risk between vaccinated and unvaccinated people. 
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(Analysis by QP Briefing based on government of Ontario data. The susceptible population is based on the number of unvaccinated Ontarians, plus a fraction of the vaccinated people, estimated as the population multiplied by one minus the vaccine effectiveness rate, 0.6–0.8. A small proportion of the unvaccinated population would have gained immunity from a prior infection but that isn't captured here and, according to Fisman, wouldn't significantly impact the results.)
What that chart tells you is that case counts are falling and the risk to Ontarians, whether they've had a shot or not, has been declining for over a month. The province introduced strict public health measures in April, turning the course of the third wave around, and vaccines are helping that along.
It also tells you that at the peak of the wave, the risk for the unvaccinated population — which was then the majority of Ontarians — was higher than the topline numbers reported by the province suggested.
The average risk for the unvaccinated population is now in what the province categorizes as the "red zone" overall, while for those who are vaccinated, the average risk has dropped to the orange or yellow zone, depending on vaccine effectiveness.
It's important to note that this doesn't take into account geographic differences — case rates and risk remain much higher for some parts of the province, for everyone but particularly the unvaccinated. Currently, the hardest-hit regions are Timmins and Peel.
According to Fisman, measuring the impact of COVID-19 on the susceptible population is especially important as Ontario moves toward lifting public health restrictions because the burden of the disease falls on them.
"Now, as more and more people in the population become immune, the overall risk in the population can continue to go down, but paradoxically, you can see risk rise among non-immune individuals," said Fisman. "You could see a rise in force of infection, and have an increasing concentration of risk in unvaccinated people, even as risk declines in the population overall because there are fewer susceptible people, there are fewer people at risk."
That is more likely to occur where there are pockets of lower vaccination rates, be they geographic or cultural communities, he said.
"We may see little clusters, little hot spots at risk, if there are neighbourhoods where there's less vaccination, if there are communities where their populations are less likely to be vaccinated," he said. "They can continue to flare even as the population overall is much less affected by transmission."
That has been playing out in the United States, where vaccine uptake has been hampered by vaccine refusal and hesitancy to a degree Canada hasn't seen, polarized along geographic and political lines, said Fisman. That has led to considerable differences in the force of infection among the susceptible populations of different U.S. states, something The Washington Post <a href="https://www.washingtonpost.com/health/interactive/2021/covid-rates-unvaccinated-people/" target="_blank" rel="noopener">explored in this feature</a>, inspiring QP Briefing's analysis.
It drives home the need to combat vaccine hesitancy and barriers to access in all communities.
Fisman noted the analysis shows residual risk among vaccinated people tracks with the risk to unvaccinated people but at a much lower level. "What that's telling you is that when you have these unvaccinated pockets, and you have ongoing transmission of infection, that's still creating risk for everyone, vaccinated and unvaccinated, because even in the vaccinated people the disease's going to find those gaps," he said.
Vaccine effectiveness
The finding of gaps Fisman refers to — the ability of the virus to infect people who have been vaccinated — is determined by the effectiveness of the COVID-19 vaccines. The greater effectiveness, the fewer vaccinated people there are among the susceptible population.
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Dr. Jeff Kwong, a scientist with Public Health Ontario and leader of the populations and public health research program at ICES, has been studying vaccine effectiveness in Ontario. It's more complex than the efficacy figures reported out from clinical trials and reflects the real-world experience playing out in the province, where multiple factors, including the timing of doses, the mix of vaccines, the COVID-19 variants in circulation and the demographics of the population, influence the outcome.
In Ontario, the interval between first and second doses has been a particularly hot topic of debate.
Kwong and other researchers looked at the effectiveness of the two most common vaccines in use in Ontario — the Pfizer and Moderna mRNA vaccines — against symptomatic infections. The results, <a href="https://www.medrxiv.org/content/10.1101/2021.05.24.21257744v1.full.pdf" target="_blank" rel="noopener">detailed in a study released as a pre-print on Friday</a>, show both vaccines are highly effective after two doses and that the effectiveness of a single shot increases significantly as time passes, from 48 per cent in at 14 to 20 days after that first needle to 71 per cent at 35 to 41 days post-vaccination. A second shot increases the vaccine effectiveness to about 90 per cent.
Likewise, protection against hospitalization and death increases significantly with time. It grows from 62 per cent at 14 to 20 days after the first shot to 91 per cent by day 35, and bumps up to 98 per cent after the second shot.
"You can see vaccine effectiveness against the severe outcomes is really quite high [at 35 days after] just one dose, comparable to receiving two doses," said Kwong. "This is quite reassuring."
There are differences among different segments of the population — Kwong noted older adults take longer on average to build higher levels of protection.
The variants of COVID-19 matter as well. The study shows the higher protection against the original strain of COVID-19 and the B.1.1.7 variant that is now dominant in Ontario but suggests lower effectiveness against symptomatic infection after only one dose with the B.1351 and P1 variants first discovered in South Africa and Brazil, said Kwong. The vaccines provided strong protection after two doses against all of those variants.
Next, Kwong is planning to study the effectiveness of the vaccines against the B.1.617.2 variant found in India, which has been found in Ontario and may be spreading. However, the province hasn't established a surveillance system for that variant, so the research will rely on an indirect method of determining cases, he said.
Other preliminary studies have suggested effectiveness against that variant is less after just one dose, he said.
"It's quite worrisome, if we're going to reopen the schools and just reopen in general," Kwong said. "What are the implications of that? We've got good one-dose coverage right now in the province. It suggests we really need to go to getting second doses into people's arms as quickly as possible so that they can have as much protection as possible."
The finish line
Fisman said the B.1.617.2 variant puts Ontario at a real risk of a setback but the finish line marking the end of the worst of the pandemic is near.
"It's just a matter of, do we want to have one last surge before we vaccinate our way out of this? My feeling is no, we sure don't," he said.
"Whether we drop the ball a fourth time or not, we'll continue to vaccinate apace and we will get there," Fisman continued. "I do think we're going to be one of the lucky places that actually attains herd immunity, at least transiently, and we're going to have to continue to adapt to a virus that obviously has an amazing ability to recombine. But I do think that we're getting towards the end of the riskiest part of the pandemic in Ontario, thanks to vaccines."

Data interactive: The unseen pandemic among susceptible Ontarians

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