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Pertussis Epidemic 2010

Bordetella pertussis is the bacterium that causes whooping cough – the main clinical feature of which is a severe lingering cough that can last for weeks or even months. Right now we are in the midst of an epidemic of pertussis cropping up in pockets throughout the US, most notable California. According to the CDC:

During January 1– June 30, 2010, a total of 1,337 cases were reported, a 418% increase from the 258 cases reported during the same period in 2009. All cases either met the Council of State and Territorial Epidemiologists definitions for confirmed or probable pertussis or had an acute cough illness and Bordetella pertussis–specific nucleic acid detected by polymerase chain reaction from nasopharyngeal specimens.

In addition, if the trends continue through the end of this year, which they are likely to do, this will be the highest incidence of pertussis in almost 50 years. These numbers are not in question, but there is some discussion about what, exactly, is causing it.

The tempting conclusion is that pertussis is making its way back into the population due largely to vaccine refusal and anti-vaccine propaganda. However, there is yet no data to support that conclusion. It may or may not be the case – we will know once a more thorough analysis is done of the individual cases of pertussis. And in any case, there are many factors at work.

First, pertussis has a natural tendency to cycle every 5 years or so, and this year is the peak of the cycle. This is certainly a significant part of the increase this year, regardless of other contributors.

In addition, the lack of vaccine-induced immunity is also playing a role, but not necessarily from vaccine refusal. Pertussis is a very contagious illness, partly because people are often contagious with it for days or weeks prior to knowing they have it, or that their cough is not just a common cold. Prior to vaccination pertussis was a significant cause of childhood death, causing about 8,000 deaths a year in the US alone. After the wide availability of vaccination against pertussis there has been on average about 10 deaths per year.

The current vaccine is an acellular pertussis vaccine (part of the DTaP injection, which included diphtheria and tetanus). The aP vaccine is a toxoid vaccine – it contains inactivated toxin proteins which are themselves harmless. It is therefore  a very safe vaccine with few side effects. Prior to 1996 the whole-cell pertussis vaccine was used – this was similar but still contained entire bacteria (although inactivated) and had a higher incidence of side effects. The DTaP vaccine is actually less effective than the older DTP vaccine, but a little bit of efficacy was traded for increased safety.

The childhood vaccine schedule requires 5 injections between 2 months and 6 years of age. Young infants are therefore most susceptible to pertussis because they have not yet had time to get vaccinated and develop immunity. Immunity does last for years, but wanes in teenagers and older adults. Therefore periodic boosters (with a vaccine called Tdap) are recommended to maintain lifelong immunity. Incidentally, immunity from the vaccine is not much different than immunity from the illness itself:

A review of the published data on duration of immunity reveals estimates that infection-acquired immunity against pertussis disease wanes after 4-20 years and protective immunity after vaccination wanes after 4-12 years.

The factors, therefore, that are contributing to the fact that the current epidemic is likely to be the biggest in 50 years are – the natural cycle of pertussis, a lower degree of immunity from the current DTaP vaccine vs the older DTP vaccine, and waning immunity in older children and adults with low rates of booster shots to maintain immunity.

Two other factors are currently under investigation. One is the rate of undocumented aliens in California that may not have been vaccinated. The CDC reports:

Incidence among Hispanic infants (49.8 cases per 100,000) was higher than among other racial/ethnic populations. Five deaths were reported, all in previously healthy Hispanic infants aged <2 months at disease onset; none had received any pertussis-containing vaccines.

So early indications are that the Hispanic population is disproportionately getting pertussis. But the burning question is – are there pockets of low vaccination rates among vaccine-refusers, lacking herd immunity, that are also contributing to the epidemic? A recent New York Times article by Tara Parker-Pope argues that vaccine refusal is likely not a contributor because there is no association between county-wide vaccine rates and pertussis incidence. However, this argument is not valid. Counties are a mostly arbitrary political boundary, not a meaningful population or social boundary. There are small pockets of low immunization rates in communities that have been centers of vaccine-preventable diseases in the past, and it is still possible (even probable) that pertussis is having an easier time spreading through these populations as well.

Further – we are on the cusp of a new school year. Once children go back to school, the pertussis epidemic may get into full swing. There are schools that, because of their culture and policies, have very low vaccination rates. We will have to see what happens with pertussis in these schools once the classroom doors open.

Conclusion

What we can say at this point for certain is that 2010 is an epidemic year for pertussis, and this cycle will be the worst in half a century. We know that vaccination with DTaP is safe and effective, but requires booster shots as adults, and that not enough people are getting this booster shot. This epidemic is still nothing compared to the pre-vaccine era of pertussis, but it highlights the ongoing need for vaccination and herd immunity against contagious and deadly diseases like pertussis.

Whether vaccine refusal is playing a significant role has neither been confirmed or rejected by current information, but eventually this data will be available. And unlike the anti-vaccine crowd, we will base our conclusions on the evidence, not rhetorical expediency.

Posted in: Vaccines

Leave a Comment (25) ↓

25 thoughts on “Pertussis Epidemic 2010

  1. CW says:

    I consider myself a pretty well-informed person, and it wasn’t until this morning that I realized adults still needed booster shots (other than tetanus).

    This website: http://www.nfid.org/pdf/factsheets/adultqa.pdf says one Td booster is sufficient (accounts for a “tetanus, diphtheria, acellular pertussis (Tdap) vaccination”).

    Are there any other vaccine boosters needed?

  2. rork says:

    My compliments to the writer, as always. I like lean.

    I was expecting a B. parapertussis versus B. pertussis discussion, perhaps calling for research on incidence rates (maybe being done already) and if the former is becoming common, to say how the process for changes in the vaccines might happen. I haven’t studied it hard, and could use some spoon feeding, and perhaps the data is small. I also wonder and worry if anti-vax folks who I do not closely follow are trying to leverage the recent paper:
    http://www.ncbi.nlm.nih.gov/pubmed/20200027, “Acellular pertussis vaccination facilitates Bordetella parapertussis infection in a rodent model of bordetellosis”, since that may seem like depleted uranium rounds if B. parapertussis is common and as nasty as B. pertussis (is that known?)

  3. tmac57 says:

    Steve:Is the acellular pertussis component a toxoid vaccine? I thought only the diphtheria and tetanus components were toxoids.

  4. art malernee dvm says:

    The DTaP vaccine is actually less effective than the older DTP vaccine, but a little bit of efficacy was traded for increased safety.>>>>

    I also did not know acellular pertussis was a toxoid vaccine. In veterinary medicine we also have a Bordetella vaccine for what is commonly called kennel cough. bordetella vaccine will not prevent disease. At best it possibly could decrease some symptoms of kennel cough.

  5. Henchminion says:

    I wonder if it’s the delivery of the vaccine that could be the problem?

    Last winter I gashed my thumb and went down to my university’s health clinic here in Toronto because I was overdue for a tetanus shot. I was given a choice of a tetanus and diptheria shot for free or a diptheria, tetanus and pertussis shot for $40. Being a starving grad student, I admit I took the cheap route.

  6. Like CW, I was unaware that adults required booster shots for this sort of thing.

    I very recently had what I thought was the common cold, though I have had a lingering cough which still remains with me over a week after recovering from the initial symptoms. My wife came down with what I thought was the same thing, though she has mostly just had a cough, and now an ear infection. Upon reading up on pertussis, apparently otitis media can also be one of the symptoms.

    Whee, looks like I should head back to the doctor to see if there is anything else that they can do.

  7. minne peds says:

    @ Tmac 57
    The TDaP has 4 components against pertussis one of which is detoxified toxoid.

    Even though we cannot automatically blame vaccine refusal for this uptick in pertussis, it does give us a great talking point when parents are on the fence. Explaining that infants actually die from whopping cough takes many of them by suprise and indeed does help get some vaccinated.

    In my practice I sense the pendulum swinging back to more vaccinators. We will have to wait and see actual data to see if that is the case but there is hope out here.

    If only we could get colloidial silver into the vaccines instead of thimerisol…….

  8. hairyape68 says:

    Two years ago, I decided to get my tetanus booster. I was surprised to learn at the health department that I would get a DT but no P, as I was too old (74) for the pertussis vaccine. I don’t think I have ever had pertussis, though I can remember my sister having it when I was very young. Is there some reason an older person can’t be vaccinated for pertussis?

  9. nybgrus says:

    Just a point of note on the anti-vaccine topic – there is as much as 10 times more mercury in an average tuna fish sandwich than there is in the thimerosol contained in any vaccine.

  10. SkepticalLawyer says:

    After giving myself a nice deep cut over the weekend, I called my doc to see when I had last had a tetanus booster. It was a little over ten years ago, so I went in for another one. Thanks to SBM, I knew to ask about the Tdap. I was glad to see that they were all ready to give it to me.

    I think I may have had a mild reaction with annoying headache last nigh, but it was worth it for the peace of mind.

  11. hokieian says:

    “My compliments to the writer, as always. I like lean.”

    I agree. I love this blog, but my one complaint is the length of the posts. I usually end up reading the first half and then only skimming the second half.

    This post was a great length. Long enough to provide enough detail, yet not overly verbose. I’d like to see more like this from SBM.

  12. Watcher says:

    I don’t know if DTaP has thimerosal in it … but, even if it did, so what?

  13. Dawn says:

    @Watcher…yes, that comment from minne peds confused me too, but I’ve been having problems signing in.

    The DTaP, which is given to children, has no thimerosal in it, according to the CDC website listing vaccine ingredients. Same for the TDaP. The only vaccines listed that have thimerosal in them are those available in multi-dose vials (except for a few flu vaccines, single dose, with trace amounts of less than 0.3 mcg)

    Why would colloidal silver be any better than thimerosal anyway?

  14. Dawn says:

    Did my comment with the URL vanish? I didn’t get a possible moderation statement so I am trying again:

    http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/b/excipient-table-2.pdf

  15. cloudskimmer says:

    Dr. Novella,

    The CDC website indicates, as you describe, cyclic variation in incidence of 3-5 years. (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5926a5.htm)

    It also says that in the last peak year (2005) there were approximately 3000 cases of the disease. Given that this year’s six month incidence is 1377, and doubling that gives me 2754, it looks like this year’s pertussis is on target for a normal peak–on target to go below that of 2005, not a 50 year high. What’s wrong with my math? Is incidence higher in the fall, so that the first six months of a year is not indicative of the last six? Is this year’s first six months increasing each month? Yes, it’s higher than 2009 by a large margin, but is this within normal range of a peak every 3-5 years? I can’t find this on the CDC website; only the comparison to last year.

    I agree with getting vaccinated, and will try to get my booster during my next doctor visit, but wonder if your assertion that this is the worst outbreak in 50 years is accurate. Why was the incidence in 2005 so high; wasn’t this before Jenny McCarthy scared people away from vaccines? Why is there a 3-5 year oscillation in incidence?

  16. minne peds says:

    My sarcasm isn’t to obvious, sorry about the confusion

    There is no thimerisol in DTaP or any childhood vaccine except multiidose vials of influenza vaccine. Neither is there colloidial silver.

    My sarcastic point was medical woo mongers seem to love metals if they are in the colloidal silver catagory and loathe them if they are in the thimerisol category. It was my poor attempt at humor.

  17. orange lantern says:

    @ minne peds: I thought it was funny. Maybe because I married a Minnesotan.

  18. Dawn says:

    @minne peds: sorry I misunderstood you. It just seemed like a strange comment and, as you pointed out, the woomongers DO love the colloidal silver.

    My humor meter must be off (wanders off to get re-calibrated).

  19. Watcher says:

    That’s why we need more emoticons!!! ;)

  20. Dawn says:

    @Watcher: More emoticons? :-)

    I’m all for that, now that my humor meter was re-calibrated. :-/

  21. The Blind Watchmaker says:

    The TdaP is indicated for one of the every-ten-year tetanus boosters. One can actually get the TdaP if it has been 2 or more years since the last Td. It is not indicated for ages over 65. I am not sure why. Grandparents take care of many infants, neither of which will likely carry sufficient immunity.

  22. Deetee says:

    @cloudskimmer

    Regarding the cycles of pertussis I would make one point – Cycles of many infectious diseases occur naturally because outbreaks boost immunity in the survivors, and herd immunity keeps a lid on further outbreaks until sufficient numbers of newly vulnerable people have accumulated to sustain a new outbreak. So in essence, keeping immunity high through vaccination will help ameliorate or eliminate the outbreaks in the first place, and so vaccination is important.

    That said, the reference you cite says this:

    The incidence of pertussis is cyclical, with peaks occurring every 3–5 years in the United States (2). The last peak was in 2005, when approximately 25,000 cases were reported nationally and approximately 3,000 cases in California, including eight deaths in infants aged <3 months. If the rates from the first half of the year persist throughout 2010, California would have its highest annual rate of pertussis reported since 1963 and the most cases reported since 1958.

    I think that because of the inherent delay in reporting (up to a few months is typical) that what they say could well be true, and numbers will exceed the 2005 peak, and this explains the apparent “defect” in the math.

  23. Prometheus says:

    For those who are curious, the acellular pertussis vaccine contains four (inactivated) pertussis proteins:

    PT, FHA, PRN and FIM

    PT is pertussis toxin and is specific to Bordetella pertussis.

    FHA (filamentous haemagglutinin), PRN (pertactin) and FIM (fimbriae) are adhesins that allow the bacteria to “stick” to ciliated epithelial cells in the nasopharynx. These proteins are found in B. pertussis as well as B. parapertussis and B. bronchiseptica, but the vaccine is prepared with only B. pertussis proteins and they appear to be sufficiently different from B. parapertussis and <B. bronchiseptica that there is no cross-immunity.

    The fact that the PT component of the acellular pertussis vaccine doesn’t “work” on B. parapertussis is irrelevant since B. parapertussis doesn’t make PT. Neither does B. bronchiseptica.

    Oddly enough, studies have shown that the older “whole-cell” pertussis vaccine did provide some protection against B. parapertussis while the acellular pertussis vaccine is pretty useless (see: David et al Vaccine 2004). There has been ongoing research into parapertussis vaccines, and an acellular version could be developed quite easily, if parapertussis became a significant problem.

    Of course, I expect the anti-vaccination movement to exploit confusion between B. pertussis and B. parapertussis by saying that the pertussis vaccine is ineffective against “this new strain of whooping cough”. In reality, B. parapertussis is not new, nor is a “recent mutation”.

    Prometheus

  24. rork says:

    Thanks Prometheus.
    They can just say new epidemic rather than new strain, not that I’ve seen good data to show that’s true.
    And though Bordetella parapertussis may not be new or a recent mutation, it can cause whooping cough. I did note at least one paper saying it was less severe (on average!) though, and then found the review paper linked below that pointed to yet others.

    I still was not able to tell myself I had a good grip on how it compares for ease of transmission or severity of illness, though I did see with relief that death from B parapertussis was deemed comparatively “rare”.

    That review paper was http://www.ncbi.nlm.nih.gov/pubmed/15831828 from Clincial microbiology reviews 2005. It has lots of interesting little tid-bits (sex ratio, US vs Europe, B parapertussis in Ohio, non-synchrony of cycles for the two species), as you might hope for in a paper of 56 pages – but don’t be scared off – the 848 references take up enough space that it seems shorter.

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