An analysis of numerous studies and reports finds that unvaccinated or undervaccinated U.S. individuals comprised a substantial proportion of cases in measles and some pertussis outbreaks. The analysis also found a generally increased risk for measles and pertussis among people who refused vaccines, as well as an associated risk among fully vaccinated individuals.

The study is published in the March 15 issue of JAMA.

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 U.S. outbreaks.

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

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 percent) 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 percent of unvaccinated individuals were intentionally unvaccinated.

"This review has broad implications for vaccine practice and policy," says Omer. "For instance, fundamental to the strength and legitimacy of 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. Similarly, central to any justification to restrict individual freedom by mandating vaccines to prevent harm to others is an understanding of the nature and magnitude of these risks and harms."

"Our report shows that outbreaks of vaccine-preventable diseases in the United States have prompted clinicians, public health officials and the public to pay greater attention to the growing phenomenon of vaccine refusal and hesitancy. We believe this study will result in an improved understanding of the association between vaccine refusal and the epidemiology of these diseases."