Fifty-nine percent of measles patients were unvaccinated despite being eligible, while 24-45% of those who developed pertussis during outbreaks were unvaccinated or under-vaccinated individuals. Those are the findings of the a new meta-analysis published in JAMA.

Researchers at Emory University in Atlanta analyzed reports of measles outbreaks that occurred since measles was declared eliminated in the U.S. in January, 2001, and reports of endemic and epidemic pertussis since the lowest point in U.S. incidence, after January 1, 1977. They identified 18 measles studies which included a total of 1,416 cases, and 32 pertussis outbreak reports which included a total of 10,609 individuals.

Nine-hundred and seventy individuals with measles cases had detailed vaccination data, and 574 of these were unvaccinated despite being vaccine eligible. The major reason for not vaccinating against measles was due to nonmedical exemptions, such as religious or philosophical reasons, as opposed to medical contraindications.

Of the pertussis outbreaks, 9 reports describing 12 outbreaks provided detailed vaccination data on unimmunized cases, and among 8 of these outbreaks, 59% through 93% of unvaccinated individuals were intentionally unvaccinated. The 5 largest statewide epidemics of pertussis had up to 45% of unvaccinated or under-vaccinated individuals.

The authors noted that “justifications to override parental decisions to refuse a vaccine for their child is a clear demonstration that the risks and harms to the child of remaining unimmunized are substantial.” They also acknowledged that the risks associated with vaccine refusal are imperfectly defined, and pointed out how, “the association between vaccine refusal and vaccine-preventable diseases may be both population- and disease-specific.”