Wednesday, August 8, 2012

Washington State Pertussis Outbreak

There is an outbreak of pertussis in Washington state that has produced a prevalence that hasn't been seen in decades.  In fact, 1942 was the last time this many cases was recorded.  So far there have been 3400 reported cases and the age groups most affected are infants less than one year old and children ages 10-13 years old.  This outbreak, as in California two years ago has demonstrated the reduced duration of efficacy and to some extent, the overall efficacy of diphtheria, tetanus and acellular pertussis toxoid vaccines (DTaP).  This has led to anti-vaxxers exploiting these outbreaks as a demonstration that DTaP vaccines are useless.  Whereas nothing could be farther from the truth.

One particularly notorious anti-vaxxer, Joseph Mercola has used the California outbreak to triumphantly announce that the majority of those infected with pertussis were fully vaccinated.  Well duh, the majority of children are fully vaccinated, but more on that later.  He also cherry-picks an editorial that was published in Pediatrics written by eminent pertussis researcher J.D. Cherry earlier this year.  Mercola only lists the contributing factors for the increase in pertussis prevalence and overestimation of vaccine efficacy but leaves out the contributing factors for underestimating vaccine efficacy and his conclusions.  For instance, Dr. Cherry writes:
Of nasopharyngeal specimens from patients with cough illnesses sent to a commercial laboratory for PCR testing, during the 3-year period 2008–2010, it was found that 14% of the positive specimens were IS1001 positive, indicating B parapertussis infection.(14) In 2010, the positivity rate was 16.5%. These cases would appear as vaccine failures when they are not, as protection against B parapertussis is not expected from current pertussis vaccines.
Vaccine use has resulted in genetic changes in PT, PRN, and FIM in circulating B pertussis strains, and it has been suggested that this has led to increased vaccine failure rates.(22) At the present time, however, there is no evidence to support the hypothesis that evolution is allowing circulating B pertussis strains to escape from vaccine-produced antibodies.(23) If it were to occur, I would expect it to occur first in Denmark, where a PT toxoid vaccine has been in use for ∼15 years; this, as yet apparently has not happened.
Given that Joe Mercola is a salesman, not a researcher and doesn't even know the difference between viruses and bacteria, I'm a bit more inclined to rely upon the expertise of a globally-recognised pertussis researcher who concludes:
To overcome the problem, it needs to be recognized that B pertussis is circulating in all age groups and, therefore, for herd immunity there is a need to universally vaccinate all age groups at frequent intervals.(24) New vaccines should be considered for development that include changes to enhance efficacy but retaining a low reactogenicity profile. This could be DTaP vaccines with multiple additional components and perhaps containing less PT. An alternative would be to develop DTP vaccines with detoxified lipopolysaccharide (the cause of reactions to whole-cell vaccines). It has also been suggested to develop “live vaccines.”(25) There are data available (not presented here), however, that indicate that immunity from DTP vaccines is better than that after infection; therefore, I do not think “live vaccines” are a worthwhile approach. Clearly, additional investments and innovations in pertussis vaccine development are needed to remove pertussis from its position as the leading vaccine preventable disease in the United States.
It is obvious from the California pertussis outbreak in 2010 and the current Washington state as well as the rest of the U.S. currently, that more effective and durable vaccines are needed, certainly not returning to the "good ol' days" of the pre-pertussis vaccine era that begot thousands of infant deaths annually.

Another fallacy that Joe Mercola propagates along with his witless followers is that more vaccinated than unvaccinated have been infected with pertussis.  When one simply looks at the raw data, it appears that way but those claiming more vaccinated than unvaccinated are being infected are either being wilfully dishonest, epidemiologically-ignorant or both.  Let's look at the raw statistics from the CDC as of 20 July 2012 and the Washington Department of Health as of 4 August 2012 (in the format presented by Medical News Today):
  • There have been a total of 3,400 cases reported statewide through week 31, compared to 287 reported cases in 2011 during the same time period. That is a 1085% increase for the same time period.
  • The overall incidence year to date is 50.5 pertussis cases per 100,000 Washington residents with a rate in infants under one year of age of 241.7 per 100,000. Two hundred and fourteen infants under one year of age were reported as having whooping cough and forty-three of them were hospitalized. Of those hospitalized, thirty-five (81%) were very young (three months of age or younger). Adolescents aged 10-13 years old also have a high incidence rate, 238.2 per 100,000, and comprise 24% of the total cases.
  • 75.8% of patients aged from 3 months to 10 years were up-to-date with their childhood diphtheria and tetanus toxoids and acellular pertussis (DTaP) doses.
  • 43.1% of patients aged 11 and 12 years and 77.2% of 13 to 19 year olds were up-to-date with their TDaP booster shots.
  • 93.2% of children aged from 19 to 35 months had received three or more doses, while 81.9% had received four or more in 2010.
  • No deaths caused by whooping cough have been reported.
Using these data, demographic/epidemiological data from Table 2. WA State pertussis cases by age group, 2012 YTD and Washington State School Vaccine Exemption data age stratified attack rates and rate ratios were estimated.  These are crude estimates as the age stratification for pertussis incidence reports and vaccination status overlapped.  The state of Washington reported a statewide 5% vaccine exemption for kindergarten through twelfth grade and this was used to estimate the un/undervaccinated populations.  Attack rates for vaccinated (ARv) and un/der vaccinated (ARn) are as follows:

Ages 5-9:    (ARv) 500/411023 x 100 = 0.12%
                     (ARn) 159/21633 x 100 = 0.73%

Ages 10-13 (ARv) 356/329076 x 100 = 0.11%
                      (ARn) 469/17320 x 100 = 2.7%

Ages 14-18 (ARv) 477/431968 x 100 = 0.11%
                      (ARn) 141/22735 x 100 = 0.62%

As we can see, more un/undervaccinated children have been infected with pertussis than fully vaccinated across all age bins.  This translates to children ages 5-9 un/der vaccinated children are 6 times more likely to become infected with pertussis than fully vaccinated. Children ages 10-13 un/der vaccinated are 25 times more likely to become infected with pertussis than fully vaccinated. And un/der vaccinated children ages 14-18 are 6 times more likely to become infected with pertussis than fully vaccinated.

Two hundred and fourteen infants under the age of one year were infected and 43 hospitalised.  Twenty four percent of those three months to one year were un/undervaccinated.  Of those hospitalised, 35 or 81% were three months or younger.  This is appalling that these children suffered needlessly; we need to do more to protect this vulnerable population and people need to stop buying into anti-vaccine lies.


  1. Excellent post. I am a physician in WA, working in a setting that is frontline in the testing for pertussis. I think it is likely that some of the increased incidence is due to increased surveillance. Screening increased dramatically this past year. That is not to say that there is not an epidemic ongoing. I think that is best reflected in the increased rates of hospitalization and morbidity and mortality. I think this has been a problem percolating in WA with the declining vaccination rates for years and has finally reached a tipping point. I place some of the blame on some of my fellow physicians and local medical groups for their ostrich -like approach to the declining vaccination rates. The lesson here is for those areas of the country that have yet reached the herd immunity tipping point: take action now.

  2. Thank you. I've been following the epidemic (I think we're beyond calling it an outbreak) for several months now, and I'm tiring of the vaccine denialists pseudoscientific analysis. Mercola's junk medicine is very amusing. I took apart his live attenuated virus recombining in the wild (though laughable his thinking pertussis is a virus). He doesn't even get basic evolutionary science.

    Loved your mathematical analysis, though if we could get the direct epidemiological information patient by patient, it would be much more robust. But your prospective analysis is way better than than the fake statistics from the denialists.

    Thanks again Science Mom.