Unvaccinated

Children

2001 Nature - Imperfect vaccines and the evolution of pathogen virulence
2001 The subsequent evolution leads to higher levels of intrinsic virulence and hence to more severe disease in unvaccinated individuals.
2015 Science - Could some vaccines make diseases more deadly?
2018 Quanta - Vaccines Are Pushing Pathogens to Evolve
COVID-19 and unvaccinated people links
In one experiment, unvaccinated birds infected with the most virulent strains were housed together with healthy birds.
The team found that, over the course of their lives, the unvaccinated birds shed far more of the least virulent strains into the environment, whereas the vaccinated birds shed far more of the most virulent strains.
Thus, "vaccination enabled the onward transmission of viruses otherwise too lethal to transmit, putting unvaccinated individuals at great risk of severe disease and death," the authors write online today in PLOS Biology.