Why the resurgence in the disease? There are several reasons, including that the bacteria that causes it (Bordetella pertussis or Bordetella parapertussis) have mutated and evolved, making traditional antibiotics less effective. But many public health officials see a much bigger problem: parents refusing to vaccinate their children.
Dr. Steven Novella, a Yale physician writing on the Science-Based Medicine blog, points out that "there are serious concerns about vaccine refusal as a contributor to the resurgence of whooping cough. Thanks to the anti-vaccine movement there is unsubstantiated fear about the safety of vaccines. In particular there are pockets of vaccine refusal resulting in a loss of herd immunity. Herd immunity results when enough of the population is immune so that an infectious disease cannot spread, so cases become isolated and do not cause an epidemic. Without herd immunity pertussis infections can spread through a population."
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Novella also refutes a claim often made by parents who refuse to vaccinate their children, that as long as other kids are vaccinated they don't need to vaccinate their own. "The evidence indicates that unvaccinated children are at higher risk of developing whooping cough than vaccinated children. Existing herd immunity is not sufficient to protect the unvaccinated, even in areas of high vaccine compliance. Further there is early evidence that whooping cough is spreading the most in counties that have a high vaccine refusal rate."