According to the study, "unvaccinated individuals comprised substantial proportions of cases in measles and some pertussis [whooping cough] outbreaks, and vaccine refusal — as measured by population-level vaccine exemption rates — was associated with an elevated risk for measles and pertussis, including among fully vaccinated individuals."

The most common form of vaccine refusal is when parents claim non-medical exemption to a school's mandatory immunization requirements, says Saad Omer, Ph.D., M.P.H, professor of global health at Emory University and senior author of the study. Non-medical exemptions are given when parents claim the vaccines conflict with their religious beliefs or go against their personal or philosophical morals, and there has been a steady increase of them in the last 20 years.

"These non-medically exempt children make up 2/3 of those who are eligible for vaccines, which includes people who can't get vaccinated for medical reasons," Omer says.