BIDDEFORD — In each of the past five years, Maine has experienced hundreds of cases of pertussis (“whooping cough”), whose terrifying nature comes from its namesake symptom: a deep, gasping cough that sounds – and feels – as though the cougher is on their last breath. Adults experience a grueling illness that tortures them with coughing so intense it often leads to vomiting for weeks to months. For children, the disease can be fatal.
A recent Press Herald story detailed the rising number of pertussis cases in York County, and quoted health experts who attributed the high numbers to an increase in unvaccinated children. While declining vaccination rates have absolutely played a role in the state’s rising rate of pertussis, they do not explain it entirely. Maine is one of several states that has seen cases of pertussis in children who are fully vaccinated, on time and on schedule. How is it possible that a disease that was once well-controlled by a vaccine has started appearing in fully immunized children? We recently completed a study that may help answer this question.
We explored the genetic diversity of Bordetella pertussis, the microbe that causes whooping cough. We looked at the pieces of B. pertussis found in the DTaP and TDaP vaccines, and studied how those pieces interact with the immune system of a vaccinated person. Our findings suggest that it comes down to Darwinism: survival of the fittest bacteria.
When we are vaccinated, our immune system mounts a response targeted specifically to the components in that vaccine. Every bacterium present with an identical copy of the vaccine component (in this case a protein called pertactin) will be killed by the immune system. If there are a few bacteria present with a slightly different version of pertactin, they become harder to kill. Over time, these strains with slightly different versions start spreading in the population and progress from being “harder to kill” to being “impossible to kill.”
The dynamics of the other vaccine components (pertussis toxin) are cloudier, but the findings regarding pertactin are clear: There are strains that can escape immunity generated by the vaccine. This could be why we are seeing cases of whooping cough in vaccinated children. Does this mean that there is no point in vaccinating? Of course not. Vaccination protects against many circulating strains, and it is plausible that the other components of the vaccine generate enough protection to lessen symptoms if infection ensues.
Pertussis is by no means a “mild childhood illness,” as it is frequently described by anti-vaccine advocates to minimize the risks one takes in leaving their children unvaccinated. Choosing not to vaccinate is not a trivial action, as the less severe symptoms a vaccinated child may have can literally be the difference between life and death. Deciding not to vaccinate is also not an action that only affects the decider. Infants too young to be vaccinated are the most vulnerable to pertussis’ worst possible outcome: a preventable death.
This raises the question of whether Maine’s rate of vaccine refusal is unrelated to our current pertussis caseload. Do our findings suggesting bacterial evolution to escape the vaccine exonerate the anti-vaccine movement? No, not even slightly. It can be argued that enough B. pertussis strains were circulating to allow vaccine resistance to emerge precisely because of the higher number of unvaccinated individuals in the community. If there is no “warm little pond” for evolution to occur, it will not occur. We feel that while these strains may have emerged eventually, the process was undoubtedly accelerated by unvaccinated carriers.
It is easy to fall victim to dogmatic thinking on the subject of vaccines. But when it comes to disease, it’s crucial that all of us, especially those of us in the health and scientific communities, continuously look at and engage in the latest research on emerging and infectious diseases. As biomedical researchers, we will continue to work hard to understand and improve vaccinations so that they can out-maneuver evolving pathogens. In conjunction, understanding how and why rates of pertussis infections are increasing helps patients, parents and clinicians make the most informed decisions possible.
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