Unvaccinated people threaten the safety of those who are vaccinated against COVID-19 even when the immunisation rates are high, according to a modelling study published on April 25.

Researchers from the University of Toronto in Canada used a simple model to explore the effect of mixing between unvaccinated and vaccinated people to understand the dynamics of an infectious disease like SARS-CoV-2. They simulated the mixing of like-with-like populations in which people have exclusive contact with others of the same vaccination status as well as random mixing between different groups.

"Many opponents of vaccine mandates have framed vaccine adoption as a matter of individual choice," said David Fisman, from the Dalla Lana School of Public Health at the University of Toronto. "However, we found that the choices made by people who forgo vaccination contribute disproportionately to risk among those who do get vaccinated," Fisman said in a statement.

The study, published in Canadian Medical Association Journal, found that the risk to vaccinated people was lower when unvaccinated mixed with unvaccinated. However, when vaccinated and unvaccinated people mixed, a substantial number of new infections would occur in vaccinated people, even in scenarios where immunisation rates were high.

The findings remained stable even when they modelled lower levels of vaccine effectiveness for the prevention of infection, such as in those who have not received a booster dose or with new SARS-CoV-2 variants. The findings may be relevant to future waves of SARS-CoV-2 or to the behaviour of new variants, according to the researchers.

"Risk among unvaccinated people cannot be considered self-regarding. In other words, forgoing vaccination can't be considered to affect only the unvaccinated, but also those around them," the authors of the study noted. "Considerations around equity and justice for people who do choose to be vaccinated, as well as those who choose not to be, need to be considered in the formulation of vaccination policy," they said.

The researchers noted that the anti-vaccine sentiment, fuelled in part by organised disinformation efforts, has resulted in suboptimal uptake of readily available vaccines in many countries, with adverse health and economic consequences.

Although the decision not to receive the vaccination is often framed in terms of the rights of individuals to opt-out, such arguments neglect the potential harms to the wider community that derive from poor vaccine uptake, they added.