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Whooping cough vaccine–facts about waning immunity

           

A new article published this week in the Journal of the American Medical Association (JAMA) by Misegades et al. analyzed a recent whooping cough (Bordetella pertussis) outbreak in California children. Misegades determined that those who had not been vaccinated against the disease were nine times more likely to get pertussis than those who had received the entire five-shot series. However, among children who were fully vaccinated, the longer it had been since their final dose of the DTaP vaccine (which protects against diphtheria, tetanus and pertussis), the higher the risk of contracting whooping cough. This is in line with the decrease in effectiveness of the vaccine that has been discussed here and elsewhere.

There is some very important data in the article by Misegades:

  1. The 2010 whooping cough outbreak in California included more than 9,000 cases, of whom 10 died, all infants.
  2. According to medical and vaccination records, 7.8% of the kids who acquired pertussis had not received any of the DTaP vaccinations. Of those who did not contract whooping cough, only 0.9% hadn’t received any DTaP vaccinations. In other words, over 99% of the kids who did not get the disease were vaccinated.
  3. Children who got all five doses of the vaccine were at the lowest risk of getting pertussis. Only 2.8% of children who contracted whooping cough had received all five doses. This compared to 17.6% of those who did not get sick and had received all five doses.
  4. And finally, with each year that passes since the fifth vaccine dose, the effectiveness of the vaccine decreases.

In an editorial in the same issue of JAMA, Dr. Eugene Shapiro states that although these results are observational, there is a “growing consensus” that the acellular pertussis vaccine is not as effective long term as the whole cell pertussis vaccine, which was replaced by the acellular version because of perceived side effects of the whole cell version. It’s rather ironic that the more powerful version of the vaccine, where evidence supports its longer lasting effectiveness, isn’t available while we deal with numerous whooping cough outbreaks. Dr. Shapiro wrote that the future strategy for pertussis vaccinations might be to give an earlier DTaP booster dose, or, better yet, to develop a new type of pertussis vaccine. In either case, more research will be required.

So what does this all mean? The effectiveness of the DTaP vaccine does wane over time, but it is clear that the risk of catching whooping cough is much less when vaccinated (even after years) than without the vaccine. And given the small, but significant, death rate of infants who catch the disease, there really isn’t a valid reason for not having children vaccinated, even if the vaccine is not as effective as we would like. Finally, regarding the myth that vaccinated children are at higher risk of pertussis than unvaccinated ones–the evidence thoroughly debunks that canard.

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