People who are unvaccinated against COVID-19 not only place themselves at greater risk of getting infected by the virus, but also increase the infection risk of those around them who have rolled up their sleeves for a jab, according to new Canadian modelling research.
The findings, published Monday in CMAJ (Canadian Medical Association Journal), suggest the choice to remain unvaccinated affects the infection risk among vaccinated people “in a manner that is disproportionate to the portion of unvaccinated people in the population,” the authors write.
The modelling showed these findings held true even when COVID vaccination rates are high, said Dr. David Fisman, the study’s lead author and professor of epidemiology at the University of Toronto’s Dalla Lana School of Public Health.
“What we see under the (modelling) conditions, which look a lot like the COVID epidemic at the moment, is the risk for vaccinated individuals is driven disproportionately by their interactions with unvaccinated individuals,” Fisman said in an interview. “This belies the idea that (vaccination) is just an individual’s personal choice that doesn’t affect anyone else.”
The findings come at a time when Ontario is immersed in a sixth wave of the pandemic, with provincewide COVID wastewater signals indicating tens of thousands of new daily infections, albeit increasing at a slower pace than a month ago.
Experts not involved with the paper said the research helps add to the body of evidence answering a critical question: what role do unvaccinated people play in the transmission of SARS-CoV-2?
David Earn, Faculty of Science Research Chair in Mathematical Epidemiology at McMaster University, said the authors examine what happens when vaccinated people interact with unvaccinated people, and vice versa, through mathematical simulations of the real world.
The authors do note that they have likely underestimated the benefit of vaccination in terms of the impact it has on preventing transmission of the virus by vaccinated but infected people.
Though the model the authors use is simplified, and doesn’t try to account for everything that happens in a population, it’s a valuable tool to understand disease dynamics, said Earn, who called the paper “compelling.”
“They are getting across … that there is risk to vaccinated populations that comes from individuals deciding not to vaccinate,” he said. “It sounds plain and obvious that would be true. But to me there is value in articulating these kinds of theoretical points clearly.”
In general, modelling is an important tool for understanding communicable diseases, said Fisman, noting this model of different “mixing patterns” helps answer the question of how these two subpopulations — vaccinated and unvaccinated — interacting within society can “impact disease dynamics and impact infection risk.”
He said the model showed that as these two groups mixed more within their own subpopulations — like with like — the proportion of people who became ill in the vaccinated groups decreased. However, the relative contribution of risk to vaccinated people due to infection from being in contact with an unvaccinated person increased.
“The vaccinated individuals actually act as a buffer so the disease transmits less readily; the more mixing there is between the vaccinated and unvaccinated, the better the unvaccinated do. But that’s at a cost to the vaccinated individuals because the more they mix with the unvaccinated, the higher their attack rate. So they’re sort of taking the hit for the team.”
Lidia Morawska, director of the International Laboratory for Air Quality and Health at Queensland University of Technology in Australia, said the findings show “that the choices of the individual have consequences not only for them, but for the community.”
“In light of this, as a society we need to consider when individuals’ choices create risks for others and what steps to take to prevent this happening,” she said.
Afia Amoako, a co-author on the paper along with University of Toronto infectious disease epidemiologist and mathematical modeller Ashleigh Tuite, said most of the analysis was done last fall, before the Omicron variant really took hold and third doses of vaccines were introduced.
“We can’t do anything about how the virus is moving, and the mutations, which are all kind of random,” said Amoako, a PhD student at the Dalla Lana School of Public Health in epidemiology,
But there’s “lots of room to build on this model,” she added. As COVID is spread through the air, getting vaccinated is about “more than individual choices, because of how much it impacts everyone,” said Amoako.
Shweta Bansal, associate professor of biology at Georgetown University, noted that COVID transmission rates are fuelled by susceptibility, or the presence of individuals who can be infected.
“So in a community with unvaccinated individuals, there is increased risk of local surges in transmission,” she said. “However, it is important to not play the blame game. Not all individuals who are unvaccinated remain so by choice.”
In response to questions from the Star, a spokesperson from Public Health Ontario said while its immunization experts haven’t investigated this specific research question, the paper’s findings align with what is known about COVID vaccines.
“The more unvaccinated people there are, the higher the risk that they can contract and transmit the virus to unvaccinated and vaccinated people, even in a population that is highly vaccinated,” the statement said.
Megan Ogilvie is a Toronto-based health reporter for the Star. Follow her on Twitter: @megan_ogilvie
May Warren is a Toronto-based breaking news reporter for the Star. Follow her on Twitter: @maywarren11
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