There has been a decline in vaccination rates in response to unfounded safety fears, but there's been a slightly different dynamic when it comes to one specific disease: whooping cough, caused by Bordetella pertussis. In areas where vaccination is erratic, like Africa, whooping cough is estimated to cause as many as 170,000 deaths annually, many of those in infants. However, in most developed nations, aggressive vaccination programs brought the disease under control by the 1970s. But, about a decade later, rates began to rise again, and there's been a bit of an argument over why.

The authors of a study that was released this week by PNAS note that there has been something unusual about the disease's resurgence. Prior to vaccinations, the disease primarily struck children under 10; since its return, however, the peak age of infection has been 13, and people in their 20s through 50s are now contracting the disease despite having been vaccinated.

The paper indicates that three explanations have been proposed for this shift: better diagnosis, evolution around the vaccine by B. pertussis, and a time lag, after which the vaccine's effectiveness gradually declines. The authors favor the last of these, since previous work has shown that vaccinated individuals, when exposed to B. pertussis, can show an increase in antibodies against it without actually developing symptoms. So, they used a mathematical model to show that a decline in vaccine effectiveness can produce the sort of behavior seen in the real world.

What actually happens is that, while vaccination rates are moderate, there's enough of the pathogen around to consistently re-expose vaccinated individuals, providing them a boost. Once vaccination becomes thorough enough, however, this opportunity for a boost vanishes, leaving children susceptible if they get exposed in later years. In their model, the researchers saw "large-amplitude multiannual cycles," meaning sporadic outbreaks of the sort that are being seen in high schools.

In response to these trends, Massachusetts has become one of the first states to start requiring a booster in high school aged children. The authors' model indicates that this should be effective at reducing the overall incidence, but will do little to protect the most vulnerable individuals: pre-vaccination infants, who may pick up the disease from their parents. The authors suggest that giving the booster to expectant parents might be a better approach if money or resources aren't available to do both.

PNAS, 2011. DOI: 10.1073/pnas.1014394108 (About DOIs).