Pertussis Booster Vaccines May Not Fight Disease Resurgence

Lara C. Pullen, PhD

January 22, 2015

Pertussis vaccine booster schedules may not be an effective strategy against the recent pertussis resurgence, according to a new modeling study. The model predicts that to be effective, a vaccine booster schedule must reflect the underlying causes of disease resurgence. Unfortunately, experts still do not fully understand the causes behind the whooping cough resurgence.

"Our results reinforce the importance of ongoing efforts to understand vaccine-derived pertussis immunity better because it is central to developing cost-effective control strategies. If the cause of the resurgence is vaccine leakiness, then no worthwhile booster strategies are able to combat this problem, pointing toward the need for new vaccines. Our findings also emphasize the need for trouble-shooting pertussis resurgence; misdiagnosis of the problem will lead to implementing economically costly control measures with little or no epidemiological gains," write Maria A. Riolo, PhD, and Pejman Rohani, PhD, from the University of Michigan in Ann Arbor.

The researchers present the results of their mathematical modeling in an article published online January 20 in the Proceedings of the National Academy of Sciences. Their model considered pertussis resurgence as a complex applied problem that is both high-dimensional and hard to predict. Their team used the model to identify a pertussis booster schedule that would achieve disease reduction at the lowest economic cost.

The team investigated four scenarios under which the available infant vaccine might fail to prevent the transmission of infection: insufficient vaccine coverage, such as that which occurs when parents opt out of a vaccination program; a low-efficacy vaccine that fails to provide protection; waning vaccine protection, such as that which occurs when initial protection wears off over time; and "leaky" vaccine protection that reduces the risk for infection but does not eliminate it completely.

Each of these scenarios pointed toward a distinct booster schedule. In other words, the pertussis resurgence mechanism was the driver behind optimization of the pertussis booster schedule.

In particular, the investigators draw attention to the leaky immunity scenario. They were unable to find a booster schedule that could compensate for leaky immunity. "If a vaccine is too leaky, the pathogen can continue to circulate in a fully vaccinated population, and you won't be able to get elimination using that vaccine alone," Dr Riolo said in a university news release. "You can still get a large reduction from pre-vaccine levels of disease, but the leakiness limits how far you can get."

The Centers for Disease Control and Prevention recommends a series of five pertussis vaccinations for children younger than 7 years. Pertussis once seemed under control, but there has been a resurgence of disease since the 1980s.

Many public health officials support the introduction of whooping cough booster shots into childhood immunization schedules, despite a poor understanding of the root cause of the pertussis resurgence. Such booster shots may be "epidemiologically ineffective and economically costly," according to Dr Riolo and Dr Rohani.

A limitation of the study, the authors note, is that they did not attempt to model the social gathering of unvaccinated individuals, asymptomatic individuals, or household structure. Instead, they used a genetic algorithm that calculated the way that evolution by natural selection would operate over diverse booster schedules. The investigators acknowledge that the genetic algorithm does not reflect much of the real-world complexity and uncertainty associated with the pertussis resurgence.

The authors have disclosed no relevant financial relationships.

Proc Natl Acad Sci. Published online January 20, 2015. Abstract

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