Unvaccinated disproportionately risk safety of those vaccinated against COVID-19, study shows

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People who have not been vaccinated against COVID-19 contribute disproportionately to the risk of infection among those who have been vaccinated, according to a new study being released as Canadians navigate a phase of the pandemic with few public-health measures remaining.

Authors of the modelling study, published Monday in the Canadian Medical Association Journal, say this increased risk undermines the assertion by some that vaccine choice is best left to individuals, and supports “strong public actions aimed at enhancing vaccine uptake and limiting access to public spaces for unvaccinated people, because risk cannot be considered ‘self-regarding.’ ”

While most governments have scaled back on COVID-19 data collection and reporting, available indicators – including wastewater surveillance – show levels of transmission across Canada higher than at any other point of the pandemic prior to this past winter’s Omicron BA.1-driven wave. Hospitalizations are also trending upward, with more people in hospital now than at any point before the winter.

Meanwhile, government-led public-health measures such as vaccine passports and mask mandates are lifting across the country, with the public being told to assess and manage personal risk.

The trio of researchers from the Dalla Lana School of Public Health at the University of Toronto created a model of a respiratory viral disease, with people represented as either susceptible to infection, infected and infectious, or recovered from infection with immunity. Those compartments were then divided into two subpopulations, with 80 per cent of people vaccinated and 20 per cent unvaccinated.

They then simulated scenarios to assume different amounts of mixing, accounting for human tendency to interact with people similar to themselves. They also adjusted values to capture the different dynamics of the Delta and Omicron variants, the latter of which was then just emerging.

Lead author David Fisman, an epidemiologist and professor at the DLSPH, said the group found that vaccination status and the way that these groups mix interact in important ways.

“In particular, when you have a lot of mixing between vaccinated and unvaccinated people, the unvaccinated people actually get protected by the vaccinated people, who act as a buffer – but that comes at a cost to the vaccinated,” said Dr. Fisman, who co-authored the study with PhD student Afia Amoako and infectious-disease epidemiologist and mathematical modeller Ashleigh Tuite.

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When the groups became more separate, final epidemic sizes decreased among the vaccinated group but increased in the unvaccinated group because of the loss of buffering through interaction with vaccinated people, the researchers found.

“The one counterintuitive finding is that even as the risk in the vaccinated drops off, the relative contribution of unvaccinated people to risk in the vaccinated actually goes up,” Dr. Fisman said.

When the groups are largely separated, the lower immunity among the unvaccinated group pushes up the number of cases directly caused by an infected individual – a measure called the reproduction number. With that group then experiencing a “roaring epidemic,” what little contact they have with vaccinated people disproportionately drives up risk, Dr. Fisman explained.

“It goes up more and more and more, the more you push the groups apart without completely separating them,” he said.

The researchers note there is ample precedent for public-health regulation that protects the wider community from the spread of communicable diseases, even if this protection comes at a cost of individual freedom. A person who refuses treatment for tuberculosis, for example, can be legally detained in hospital to protect the public.

“We also note that the use of legal and regulatory tools for the prevention of behaviours and practices that create risk for the wider public also extend beyond communicable infectious diseases, such as statutes that limit indoor cigarette smoking,” the authors write.

Vardit Ravitsky, a professor of bioethics at the University of Montreal and Harvard Medical School, said the study adds further support to the rationale behind many of the public-health measures that had been in place, such as vaccine passports for discretionary settings.

The ethics of various measures, she said, involves an analysis of burden and benefit, of carrot versus stick, and depends on the current situation: How much virus is circulating in the community? How much risk does this pose to the public? Is the health care system able to respond sufficiently? What is the cost to individual freedoms?

In some scenarios, it may no longer be ethically justifiable to mandate certain measures if they are outweighed by the cost to individual freedoms, said Dr. Ravitsky, who is also president of the International Association of Bioethics.

“But there’s a third dimension of balance, and that is the more you give back freedom from burden to those of us who are young and able-bodied and can cope well with infection and can get vaccinated, the more we’re creating a society that is unsafe for the most vulnerable, the elderly, those with co-morbidities, immunocompromised, those who cannot be vaccinated for medical reasons, young children,” she said.

“We are, once again, turning the dial to increase the freedom for the majority but increase the burden and the risk and the worry for the vulnerable, which is a minority. And that is a value-based decision that we make as a society.”

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