03.15.2016

Pertussis outbreaks, however, have more complex origins

by Molly Walker
Contributing Writer

More than half of U.S. measles cases since 2000 occurred in unvaccinated individuals, most of whom had cited nonmedical reasons for avoiding immunization, according to a meta-analysis on measles and pertussis outbreaks.

Analysis of 1,416 reported measles cases found that 56.4% involved individuals with no history of receiving measles vaccine, whereas 14.1% involved vaccinated individuals, reported Varun K. Phadke, MD, of Emory University in Atlanta, and colleagues.

Moreover, among 574 unvaccinated age-eligible individuals with a recorded reason for exemption, 70.6% had a nonmedical exemption, they wrote in the Journal of the American Medical Association.

Yvonne Maldonado, MD, of Stanford University in Palo Alto, Calif., who was not involved in the study, said it underscores the impact of poor vaccination coverage and vaccine refusal.

“It is important to understand and mitigate factors which can affect transmission of vaccine preventable diseases in the U.S. and elsewhere,” she wrote in an email toMedPage Today. “In particular, the impact of unvaccinated and undervaccinated populations, such as those related to vaccine refusal, on population herd immunity is not always readily known.”

Phadke’s team also examined 145 cases from 18 outbreaks with enough data to calculate a cumulative epidemic curve. These included 111 unvaccinated individuals, 23 with at least one dose of vaccine, and 11 with unknown vaccination status. When viewed by week of outbreak, unvaccinated individuals comprised a larger portion of total measles cases in the earliest weeks of an outbreak.

“The availability of vaccine exemptions varies among the fifty states and is constantly the target of those who would broaden them, easing the opportunities for refusal,” wroteSamuel Katz, MD, of Duke University in an email to MedPage Today. He cited “a daily accounting of the problem” through a couple recent articles — from Colorado’s attempt to track unvaccinated children through a database to Kansas trying to increase HPV vaccine uptake among teenagers.

While recognizing that the risks of vaccine refusal remain “imperfectly defined,” Phadke and colleagues also added that their review has broad implications for vaccine practice and policy.

“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,” they wrote. “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.”

Pertussis Outbreaks: Vaccination Issues Continue

Phadke’s team also examined pertussis outbreaks in the U.S. Unlike measles, they noted, pertussis remains endemic to this country. While there has been a recent increase in pertussis outbreaks, the problem is more complex and cannot merely be attributed to lack of vaccination — although it does appear to play a role.

Problems with waning immunity from the acellular pertussis vaccine are well-documented, including recent outbreaks among younger children and a report suggesting the Tdap booster only protects older children for a shorter period of time. In fact, Phadke’s team cited a recent meta-analysis suggesting that the odds of acquiring pertussis increase 1.33 times for each additional year since the last dose of DTaP.

The authors examined 32 reports of pertussis outbreaks with a total of 10,609 cases (age range 10 days-87 years). Because the Advisory Committee on Immunization Practices(ACIP) recommends five DTaP doses and a Tdap booster, undervaccination is a concern. In the 2014 pertussis outbreak in California, of the 222 cases among infants under the age of 12 months, 24% received any doses of DTaP, even though half (51%) were age-eligible to receive it.

Several studies included in the meta-analysis had examined the impact of vaccine refusal on pertussis risk. Observational studies showed that communities with high rates of vaccine exemption in Colorado, Michigan, New York, and California were associated with an increased risk of pertussis compared to communities with a lower exemption rate. One study found states with so-called personal belief exemptions or easier exemption allowances had an incidence of pertussis up to 1.5 times higher than states with more difficult exemption processes.

The authors note that waning immunity from pertussis vaccine cannot explain this association, because “there should not be geographic heterogeneity in the duration of protection afforded by pertussis-containing vaccines.”

Opportunities for Increasing Vaccine Uptake

The authors cited several limitations to their study, including that a formal assessment of study quality was not performed and several studies on measles and pertussis disease risk could be subject to important biases such as those found in case-control or cohort studies.

An accompanying editorial by Matthew M. Davis, MD, of the University of Michigan in Ann Arbor, suggested ways to reduce exemptions and waning immunity and achieve more complete vaccination of children and adults. In addition to stricter exemption regulations across states, Davis also said efforts need to be made to adjust intervals between vaccine doses.

“Additional research could determine whether the pertussis booster interval (currently 5-10 years) should be revised to a shorter interval,” he wrote. “Similar strategies may provide opportunities for pharmaceutical manufacturers to identify markets for new vaccines while also advancing population health.”

Davis also asserted that the policy focus on vaccine refusal should include adults as well, since lack of vaccination in adults has the potential to impact the “susceptible children” with whom they come into contact.

Phadke’s team agreed, noting that reasons for vaccine hesitancy — including parental perceptions about the risk and severity of vaccine-preventable diseases versus risks of vaccines, and lack of confidence in medical professionals, and the healthcare system — should be addressed directly.

“Recognizing the limitations of the existing evidence should not stifle practitioners’ and policy makers’ ability to counsel families and craft effective policy, but should serve as motivation to develop, refine and improve disease surveillance, detection, and outcomes-based research,” they concluded.

• Reviewed by F. Perry Wilson, MD, MSCEAssistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner

LAST UPDATED 03.16.2016