Unvaccinated or undervaccinated people in the United States comprised a substantial proportion of cases in measles and some pertussis outbreaks, according to a study published recently in the Journal of the American Medical Association. 

Saad Omer, MBBS, MPH, PhD, professor in the Hubert Department of Global Health at Emory’s Rollins School of Public Health, and team examined the association between vaccine delay, refusal, or exemption and the epidemiology of measles and pertussis, two vaccine-preventable diseases with recent outbreaks in the United States.

The researchers searched the medical literature for reports of measles outbreaks that have occurred since measles was declared eliminated in the United States (after Jan. 1, 2000), endemic and epidemic pertussis since the lowest point in pertussis incidence (after Jan. 1, 1977), and for studies that assessed disease risk in the context of vaccine delay or exemption.

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Among 32 reports of pertussis outbreaks, which included 10,609 individuals for whom vaccination status was reported (ages 10 days to 87 years), the five largest statewide epidemics included substantial proportions (24 to 45%) of unvaccinated or undervaccinated individuals. 

However, several pertussis outbreaks also occurred in highly-vaccinated populations, indicating waning immunity. Nine reports (describing 12 outbreaks) provided detailed vaccination data on unimmunized cases; among eight of these outbreaks, 59 to 93% of unvaccinated individuals were intentionally unvaccinated.


1. Phadke VK, Bednarczyk RA, Salmon DA, et al. Association Between Vaccine Refusal and Vaccine-Preventable Diseases in the United StatesA Review of Measles and Pertussis. JAMA. 2016; doi:10.1001/jama.2016.1353.