How Much Vaccination Is Too Much Vaccination?

Knowing when—and not just how—to vaccinate is important in fighting off bacteria, research shows.
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(Photo: U.S. Army Corps of Engineers Europe District/Flickr)

(Photo: U.S. Army Corps of Engineers Europe District/Flickr)

Outbreaks of Bordatella pertussis, the bacteria that causes whooping cough, have been on the rise in past decades, and much of the blame has fallen on the anti-vaccination movement. While record-low vaccination rates for pertussis are partly to blame for recent outbreaks, the situation is more complicated than that. So, it turns out, is the appropriate response.

Perhaps because pertussis outbreaks are rare, we tend to assume that those who contract the disease never got vaccinated, but that’s really just one possibility. The vaccine currently in use actually loses effectiveness over time, and even if it fights off illness, it may not prevent someone from getting infected and spreading bacteria. No vaccine is 100 percent effective, either. A pertussis outbreak in Canada in the late 1990s, for example, resulted from a vaccine that just didn’t work very well.

While record-low vaccination rates for pertussis are partly to blame for recent outbreaks, the situation is more complicated than that. So, it turns out, is the appropriate response.

Obviously, policymakers want to know which of those explanations is behind recent outbreaks, since that will inform debates over school vaccination requirements and the development of new vaccines. Less obvious, Maria Riolo and Pejman Rohani at the University of Michigan argue, is that it also affects when kids and adults should get pertussis booster shots. To illustrate that idea, they considered several hypothetical scenarios that could explain the rise in pertussis cases—70 percent vaccination rates in infants, a 70 percent effective vaccine, and a vaccine that protects for 15 years on average. They then used computer simulations to determine the most cost-effective booster schedule in each scenario.

That booster schedule varied considerably by scenario. In a world with low vaccination rates, the simulations showed that giving around half of all children booster vaccines some time before they entered preschool—when they come into frequent contact with other kids—was the best strategy. The same held basically true when the initial vaccination wasn’t that effective. But preventing outbreaks due to waning immunity called for something different. That case, Riolo and Rohani write, requires three booster shots between ages six and 19, for upwards of 80 percent of children and adolescents, with additional boosters for about a quarter of the population between ages 25 and 45. There was, however, no effective booster strategy in a scenario where a vaccine protected only against certain pertussis strains, and not others.

Those findings may or may not apply in the real world, where many different factors contribute to current pertussis caseloads, but the results still illustrate the important role that the mechanism of disease transmission plays. “Our findings also emphasize the need for trouble-shooting pertussis resurgence,” Riolo and Rohani write, noting that “misdiagnosis of the problem will lead to implementing economically costly control measures with little or no epidemiological gains.”

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