Unvaccinated People Increase Risk Of Covid Infection Among Vaccinated, Study Finds

Unvaccinated People Increase Risk Of Covid Infection Among Vaccinated, Study Finds

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Unvaccinated people threaten the safety of vaccinated people even with high levels of immunization against Covid-19, according to a new study published in Canadian Medical Association Journal (CMAJ), countering a popular narrative that vaccination is purely is a matter of individual choice and supporting policies requiring the shots.

Unvaccinated people pose an increased risk of Covid infection to vaccinated people, a study found.

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Key Facts

Unvaccinated people pose a “disproportionate” risk of Covid-19 infection to vaccinated people, according to the peer reviewed study from University of Toronto researchers, which examined the interactions between unvaccinated and vaccinated groups using a computer model.

The model simulated the spread of Covid-19 under a variety of different parameters and mixing between unvaccinated and vaccinated people, including vaccine effectiveness and uptake, baseline levels of immunity among the unvaccinated and the rate of recovery from infection.

The model revealed a “markedly higher” risk of infection in all scenarios where unvaccinated and vaccinated people mixed, even the ones where immunization rates were high.

The findings counter the common argument that the decision to get vaccinated is a personal one, the researchers said, as the unvaccinated are ”likely to affect the health and safety of vaccinated people in a manner disproportionate to the fraction of unvaccinated people in the population.”

Key Background

While the risks of forgoing vaccination are greatest for the unvaccinated—they are more likely to be infected, hospitalized and die from Covid-19 than vaccinated people—the findings dent a commonly-used argument that it is the refuser alone who bears the consequences of this decision. Given the disproportionate levels of risk the unvaccinated pose to the vaccinated, vaccination cannot be framed as an entirely “self-regarding” choice, explained study author Dr. David Fisman, a professor of epidemiology at the University of Toronto. In light of this, Fisman said policies like vaccine mandates or vaccine passports that restrict access to non-essential services such as restaurant dining or public transport seem “reasonable.” Such policies are also in line with other regulations designed to protect public health, he said, pointing to compulsory treatment for tuberculosis and restrictions on driving while intoxicated. There is a strong history of law and policy supporting vaccine mandates for the sake of public health in the U.S. as well, with a Supreme Court precedent favoring mandatory smallpox vaccination dating back over a 100 years.

Crucial Quote

“Vaccinated individuals have a right not to have their efforts to protect themselves undermined,” Fisman said, stressing that the findings are “very supportive” of vaccine mandates for flights and trains.


The paper does not capture another series of health risks the unvaccinated impose upon the vaccinated, Fisman said. As the unvaccinated are more likely to end up in hospital with Covid-19, Fisman said they take up more resources and deprive the “vaccinated of access to care for other conditions” such as cancer or heart disease.

Further Reading

By The Numbers: Who’s Refusing Covid Vaccinations—And Why (Forbes)

Unvaccinated More Likely To Get Long Covid—And Suffer Symptoms For Longer—Studies Find (Forbes)

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