A few years back, I talked quite a bit about how pertussis (aka whooping cough) was becoming more common again and what the reasons for it might be. Back then, the reason du jour in the mainstream media and certain parts of the autism world for the epidemic was that the more and more parents were refusing to give their children the pertussis vaccine which was leading to a decrease in herd immunity and allowing pertussis to make a come back. This was the reason used in spite of the fact that there was evidence, even back then, that other factors played a larger role in the pertussis epidemic.
Well, lets fast forward to today and a study1 that was just published in the New England Journal of Medicine that suggests that the main cause of the pertussis epidemic is that the vaccine induced immunity doesn't last nearly as long as was previously thought.
The old idea was that the vaccine induced immunity gradually faded over the years so that a person who was fully vaccinated would only need a booster shot every 8-10 years or so to maintain their immunity.
The new idea is that the vaccine induced immunity starts fading quickly after the last vaccine dose. According to the study, the odds of contracting pertussis increased an average of 42% per year. So instead of needing a booster in ten years to maintain immunity, this finding suggests that the immunity is almost gone ten years out.
But that ten year figure assumes that the vaccine is at least 97% effective in the first year. If, as other recent results have suggested2, the vaccine is much less effective than previously though, the immunity would fade even faster. If the vaccine were 90% effective to start that would mean the protection would be cut in half in about four years and largely gone in six years.
These numbers are just approximations and are going to vary based on a large number of factors but I think the overall point is clear - the current pertussis vaccine isn't nearly as long lasting as was previously thought. The data from the outbreaks going on around the country also supports this idea. I'm not going to go into details but, as an example, roughly one third of the cases in outbreak in California in 2010 were in mostly vaccinated children from 5 to 7.
So the main reason that pertussis is becoming more common isn't that parents are refusing the vaccine but rather that the vaccine doesn't work as well as it was thought to. While this reinforces the idea that vaccines play a critical role in protecting the population from serious illnesses it also serves as a reminder that vaccines aren't sacrosanct. A vaccine should not be assumed to be safe and effective just because it is on the vaccine schedule.
This also serves as a reminder that we should wait for actual data and not jump to conclusions.
If you are interested in further reading on the subject there is the study abstract here and there are articles in Time, Web MD, The Wall Street Journal, ABC News, and CBS News.
References
1. Klein NP, Bartlett J, Rowhani-Rahbar A, Fireman B, Baxter R. Waning protection after fifth dose of acellular pertussis vaccine in children. N Engl J Med. 2012 Sep 13;367(11):1012-9.
PubMed PMID: 22970945. DOI: 10.1056/NEJMoa1200850
2.Witt MA, Katz PH, Witt DJ. Unexpectedly limited durability of immunity following acellular pertussis vaccination in preadolescents in a North American outbreak. Clin Infect Dis. 2012 Jun;54(12):1730-5. Epub 2012 Mar 15.
PubMed PMID: 22423127 DOI: 10.1093/cid/cis287
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