Mixing with unvaccinated increases COVID-19 risk for vaccinated: Study

People who are unvaccinated against COVID-19 are disproportionately more likely to infect those who have been vaccinated, according to a new study.

The Canadian Medical Association Journal is out with new modelling research and the study’s lead author, Dr. David Fisman of the University of Toronto’s Dalla Lana School of Public Health, says what they’re seeing is the risk for vaccinated individuals is being driven up by their interactions with those who have not gotten a shot.

Fisman says the message of the study suggests the choice to get vaccinated can’t be thought of as merely personal.

“Public health is something you actually have to do collectively,” he says. “What we kind of concluded is that the decision to not be vaccinated — you can’t really regard it as a self-regarding risk (because) you’re creating risk for other people around you by interacting with them.”

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In order to conduct the study, the researchers created a model of infectious disease by separating people into two subpopulations, those who were vaccinated and those who were not.

They then “simulated a spectrum of patterns of mixing between vaccinated and unvaccinated groups that ranged from random mixing to complete like-with-like mixing.”

Like-with-like mixing is where people only exclusively contact individuals with the same vaccination status. The researchers evaluated the virus within each group and within the entire population.

The results showed that infection rates were higher among the unvaccinated subpopulation but also found that group contributed to infections in the vaccinated group at a higher rate “than would have been expected based on contact numbers.”

Results also showed that infection rates decreased from 15 per cent to 10 per cent when like-with-like mixing increased among vaccinated people. Infection rates among unvaccinated people increased from 62 per cent to 79 per cent with increased like-with-like mixing.

The data suggests the unvaccinated group actually get protected by those who’ve got the shot when there is an increase of mixing between the two groups.

Simulated epidemics for different levels of mixing between vaccinated and unvaccinated populations

Source: The Canadian Medical Association Journal

Researchers say results undermine anti-vaccine arguments, supports vaccine passports

The researchers say this increased risk undermines the message that vaccine choice is best left to the individual, and instead supports enhancing vaccine uptake and limiting access to public spaces for the unvaccinated.

The authors suggest the results of the study stand in stark contrast with the anti-vaccine sentiment that not getting the shot is aligned with the rights of the individual. The researchers contend those arguments fail to consider the impact on the wider community.

“You may like to drive your car 200 kilometres an hour and think that’s fun, but we don’t allow you to do that on a highway partly because you can kill and injure yourself, but also because you’re creating risk for those around you,” said Fisman.

He argues the results of the study and the negative impacts on those who choose to get the shot would support public health restrictions for those who are unvaccinated.

The authors of the study cited indoor cigarette smoking and driving under the influence restrictions as examples where “behaviours that create health risks for the community as well as individuals have been the subject of public health regulation.”

Ontario lifted its proof of vaccination requirements in most settings on March 1. Businesses may continue to require proof of vaccine if they choose to do so.

With files from The Canadian Press