March 22, 2016
2 min read

Intentional vaccine avoidance causes most measles cases

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The majority of measles and pertussis cases during outbreaks with comprehensive vaccine data were a result of intentional vaccine refusal, according to recent research in JAMA.

“Although previous studies demonstrated associations between rates of nonmedical exemption and the risk of acquiring vaccine-preventable diseases, they … did not systematically examine measles and the contribution of refusals to disease outbreaks,” Varun K. Phadke, MD, of the division of infectious diseases at Emory University, and colleagues wrote. “This review evaluates the available evidence to characterize the relationship between vaccine refusal and the epidemiology of measles and pertussis.”

Varun Phadke

Varun K. Phadke

The researchers reviewed studies published before November 2015, which reported on measles and pertussis outbreaks in the United States. To determine the disease risk in the post-elimination era, the researchers included all reports that examined measles outbreaks after the declared elimination of measles in 2000. Likewise, pertussis outbreak reports were included after the lowest recorded level of pertussis prevalence in 1977.

Of the eight outbreaks with detailed vaccination data on unvaccinated patients, the proportion of intentionally unvaccinated patients ranged from 59% to 93%.

Among the 18 measles studies identified, there were 1,416 measles cases. Study results showed that 56.8% of measles cases had no history of vaccination. Furthermore, of the 970 measles cases with comprehensive vaccination data, 574 vaccine-eligible patients were unvaccinated, and 405 had nonmedical exemptions.

The researchers also identified 32 reports of pertussis outbreaks, including 10,609 pertussis cases with vaccine data. The proportion of unvaccinated or undervaccinated patients ranged from 24% to 45% during the five largest statewide outbreaks. Phadke and colleagues noted that pertussis outbreaks also occurred among vaccinated individuals, indicating waning of pertussis-protecting vaccine components.

“Vaccine refusal-specific strategies to optimize vaccine uptake could include state or school-level enforcement of vaccine mandates, or increasing the difficulty with which vaccine exemptions can be obtained,” Phadke and colleagues wrote. “At the same time, immunization policymakers must also address the reasons for vaccine hesitancy, which may include parental perceptions regarding the risk and severity of vaccine-preventable diseases, the safety and effectiveness of routine immunizations, and confidence in medical professionals, corporations, and the health care system.”

In a related editorial, Matthew M. Davis, MD, MAPP, of the child health evaluation and research unit in the department of pediatrics and communicable diseases at the University of Michigan, wrote that pertussis cases caused by unvaccinated individuals and waning pertussis immunity are both symptoms of the same problem.

“Although vaccine refusal and waning immunity are distinct challenges, they are also related,” Davis wrote. “Nonmedical exemptions for childhood vaccination reduce overall community immunity, thereby increasing risk of infection for individuals with waning immunity or no immunity whatsoever. When vaccine-preventable disease is diagnosed among individuals whose vaccine-acquired immunity has waned, this raises questions among parents about vaccine benefits and may inappropriately reassure parents who are skeptical about the importance of vaccination for their children. Addressing these related phenomena and how they may be misconstrued may help achieve a more knowledgeable and engaged public, thereby reducing vaccine-preventable diseases.” – by David Costill

Disclosures: Phadke and Davis report no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.