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People who are not vaccinated against a respiratory virus such as SARS-CoV-2 present a disproportionate infectious risk to those who are vaccinated, according to a mathematical modeling study.
The study, which simulated patterns of infection among vaccinated and unvaccinated populations, showed that as the populations mixed less, attack rates decreased among vaccinated people (from 15% to 10%) and increased among unvaccinated people (from 62% to 79%). The unvaccinated increasingly became the source of infection, however.
"When the vaccinated and unvaccinated mix, indirect protection is conferred upon the unvaccinated by the buffering effect of vaccinated individuals, and by contrast, risk in the vaccinated goes up," lead author David Fisman, MD, professor of epidemiology at the University of Toronto's Dalla Lana School of Public Health, told Medscape Medical News.
As the groups mix less and less, the size of the epidemic increases among the unvaccinated and decreases among the vaccinated. "But the impact of the unvaccinated on risk in the vaccinated is disproportionate to the numbers of contacts between the two groups," said Fisman.
The study was published online April 25 in the Canadian Medical Association Journal.
Relative Contributions to Risk
The researchers used a model of a respiratory viral disease "similar to SARS-CoV-2 infection with Delta variant." They included reproduction values to capture the dynamics of the Omicron variant, which was emerging at the time. In the study, vaccines ranged in effectiveness from 40% to 80%. The study incorporated various levels of mixing between a partially vaccinated and an unvaccinated population. The mixing ranged from random mixing to like-with-like mixing ("assortativity"). There were three possible "compartments" of people in the model: those considered susceptible to infection, those considered infected and infectious, and those considered immune because of recovery.
