A crowd enjoys music at a festivalShare on Pinterest
Experts say COVID-19 risk increases when unvaccinated people mingle with vaccinated individuals. Klaus Vedfelt/Getty Images
  • Researchers say unvaccinated people increase the risk of COVID-19 for everyone when they mix with vaccinated individuals.
  • Experts say the findings are important because they demonstrate how the choice to remain unvaccinated affects everyone in a community.
  • Experts say vaccinated people should consider continuing to wear a mask in certain public places, especially if they are older, have underlying conditions, or take care of someone who is at higher risk.

What happens when unvaccinated people mingle with those who are vaccinated?

Experts say the unvaccinated not only are at higher risk of contracting COVID-19, but they also pose a risk to the vaccinated, even in places where vaccination rates are high.

In a new study, researchers at the University of Toronto in Canada say that when they simulated mixing unvaccinated people with vaccinated individuals, a substantial number of new cases would occur among the vaccinated crowd.

Why is that finding important?

Dr. David N. Fisman, a professor in the Division of Epidemiology at the Dalla Lana School of Public Health at the University of Toronto and co-author of the study, says it’s important in light of the “My body, my choice argument” going on in the public.

“We’re just trying to use the model as a quantitative tool to show people modern infectious disease models,” Fisman told Healthline. “We show that the decisions people make don’t just affect their own health, it affects the health of others, those around them.”

“The problem with communicable disease is… your risk is not in your hands” he added. “That’s why historically you’ve had a public health bureaucracy… That’s been a top-down approach because you need collective action to keep everybody safe.”

“This modeling study is not surprising and we’ve seen this phenomenon in real life,” said Dr. Amesh Adalja, an infectious disease specialist and assistant professor at the Johns Hopkins Bloomberg School of Public Health in Maryland.

“The less interaction the unvaccinated have with the vaccinated, the lower the risk of breakthrough infections,” Adalia told Healthline.

“With vaccines such as the first generation COVID vaccines that do not provide complete sterilizing immunity — meaning people can get infected but at a lower rate — the unvaccinated can drive breakthrough infections in the vaccinated when they interact with them,” he added.

Dr. William Schaffner, a professor in the Division of Infectious Diseases at the Vanderbilt University School of Medicine in Tennessee said the study is a model, but the takeaways can be “very, very instructive.”

“We know that the virus can infect the vaccinated as well as unvaccinated people… but this model indicates, really with some clarity, that unvaccinated people not only have obviously increased risk for themselves… but are more efficient transmitters… drivers or engines of transmission in the community,” Schaffner told Healthline.

The Canadian study did not look at what impact proper masking might have.

However, Adalja says we already have a model. Just look at the medical profession.

“Masks, especially N95 or their equivalent, can mitigate this risk for those who are trying to avoid infection,” he said. “This is true for one-way masking, which is what healthcare workers routinely do.”

The masking debate is back. Last week, a federal judge struck down the Centers for Disease Control and Prevention travel mask mandate. The Justice Department is appealing that decision.

A recent Associated Press poll shows a slight majority of Americans, 56 percent, prefer to keep masking up on planes, trains, and buses.

Schaffner says masking may still be the right choice for some people.

“Ask yourself who you are… Am I old? Am I frail? Do I have underlying illnesses – heart disease, lung disease, diabetes? All of these diseases that predispose you to more severe disease,” Schaffner said. “And if I were in one of those categories… I would certainly prefer to wear a mask in many congregate circumstances.”

“Then there are immunocompromised people… They and their doctors know they should be completely vaccinated and also mask,” Schaffner said.

“And the other group is caregivers,” he added. “They may be healthy but may be reliant caregivers for people in those high risk groups.”