Pertussis is a highly contagious and potentially deadly respiratory infection that affects mostly children in the developing world. The low prevalence of pertussis in industrialized nations is partly due to the effectiveness of the pertussis vaccine. However, several studies have indicated that the rates of pertussis in the US have significantly increased during the last decade, likely due to a parallel decrease in vaccinations, as parents have become increasingly worried about vaccine safety. But is this true? Are children of parents who decline the pertussis vaccine at higher risk for acquiring this disease?

The journal of the American Academy of Pediatrics recently published a study that examined the association between vaccination refusal and pertussis infections in Colorado. In this study, the authors conducted a case-control examination of children between 2 months and 18 years who were members of a large health plan (Kaiser Permanente Colorado). The study included 156 cases of pertussis and 595 control non-pertussis children.

The results:

  1. The authors found time trend in the cases of pertussis, with a large increase in cases of pertussis after the year 2000.
  2. Of the 156 children who acquired pertussis, 12% were children of parents who refused vaccination.
  3. In contrast, only 0.5% of the children who did not get pertussis were children of parents who refused vaccination.
  4. Among children of all ages, refusing vaccination increased the risk of pertussis infection by 2,280%!!!

And these numbers may actually be an underestimate of the real risk of vaccine refusal, since the authors found a possible bias in diagnostic practices. Specifically, parents who agreed to vaccinate their children were 2 times more likely than vaccine-refusing parents to visit the doctor for upper respiratory infections. One interpretation of this finding is that parents who accepted the vaccination are more likely to seek medical services when their kids are sick than parents who refused vaccination. If this is the case, it is likely that cases of pertussis in the unvaccinated kids may have been missed (as these kids were less likely to visit the doctor) so that the rate of pertussis among unvaccinated kids is actually higher than what was observed in this study.

In sum, this study indicated that children of parents who refused vaccination were over-represented among cases of pertussis in Colorado during the past decade and that vaccination refusal increased the risk of pertussis by more than 2000%

These findings made me think about the underlying reasons that drive some parents to refuse vaccination: the fear that by vaccinating their kids they are increasing their kids’ risk of having a severe side effect. But I’m wondering if these parents are properly weighting the risks. Specifically, I wonder how many of these parents have reliable information about both sides of the equation; so that the real and known risks of vaccination are properly weighted against the real and known risks of non-vaccination (such as a 2,000% increase in the risk of acquiring pertussis)?

Glanz, J., McClure, D., Magid, D., Daley, M., France, E., Salmon, D., & Hambidge, S. (2009). Parental Refusal of Pertussis Vaccination Is Associated With an Increased Risk of Pertussis Infection in Children PEDIATRICS, 123 (6), 1446-1451 DOI: 10.1542/

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8 Responses to Vaccination refusal increases risk of pertussis by 2,000%: Balancing the risks of vaccines and preventable diseases

  1. K. Allred says:

    This study becomes seriously flawed when the ages of vaccinated kids AND the ages at which the kids were vaccinated range from 2 months to 18 years. There’s a HUGE difference between the side effects of a vaccination on a 2 month old and an adult child of 18 years! Parents are finally waking up to the “blind leading the blind syndrome” in American pediatrics of allowing their newborns and pre-toddlers to be vaccinated as early as 4 weeks of age. There are too many serious side effects, many with long-term damage, to infants. If the study were talking about children ages 2-18, that would be a completely different ball of wax.

    As an educator of autistic individuals, I have heard too many testimonials from parents whose infants lives changed dramatically after receiving before age 18 months vaccinations deemed “safe” by the American Pediatics Assoc. Therefore, I now ask my family members to consider postponing newborn vaccinations by twice the length of time during the infants first sensitive 18 months. What kind of statistics are available that would refute that argument?

    Any study that bases its results on a long-standing HMO is going to get results that are both flawed and suspect. Most such organizations are currently dealing with thousands of outstanding medical lawsuits. The APA would do well to mine their results from better fields.

  2. K. Allred says:

    One more thing: the study talks about parents who “refuse” vaccinations. Does this include cautious parents who merely DELAY vaccinations? If so, the that is a skewed view of the sampling.

  3. MJ says:

    There were two questions that nagged at me after reading this study. The first problem was the the numbers of refusers was very small relative to the population as a whole so a small difference in the number of refusers would have an outsized impact on the relative risk – if you take away one or two members of the refusers group the percentage would have a large change.

    Second and more importantly, the authors do not provide any details about the ages of the various groups when they contracted pertussis. If you look at some of the recent numbers published by the CDC you see that the increase in cases is mainly in older children where you would expect the protection from the vaccination to be wearing off. It would have been interesting to see a break out of the ages of the refusers vs non-refusers to see if the cases were at older ages were vaccination status would not be as important.

    You also say “I’m wondering if these parents are properly weighting the risks.” but I am not sure what that means in practical terms.

    Most of the information that is available to parents is of the variety that downplays any potential risks and plays up the fears of catching whatever the vaccine is trying to prevent.

    If you take the time and effort to look into the issue you don’t get much further – there is no real transparency into how often side effects occur or how severe the problems or what the chance of catching the particular condition is or what the condition can cause. In in simple terms there is very little that gives you a true feeling for the benefits and the risks associated with a particular vaccination.

    Without this information how is a parent supposed to properly weigh the risks?

  4. JulieL says:

    I suggest further reading for stats and vacs: “Autsim’s False Prophets” by Dr. Offit and also “Unstrange Minds” by Roy Grinker. Both books do a nice job relaying the dichotomy between science and the public. They show how statistics aren’t always as what is harped by the media. MJ, if you do some digging around, you can, as a lay person, see the lines between bad information and true scientific information. It is hard though, when the best selling books and magazines geared towards parents are from the other side of the fence. This is no fault of one person, nor implies lack of education, just points the muggy waters that are out there right now.


  5. carissa says:

    If only 12% of the children who got the disease were unvaccinated that means that 88% were vaccinated children. Seems to me that the vaccine isn’t working. I love how they can spin numbers to mean what ever they want.

  6. Jen says:

    I’m confused.

    Based on result number two, 19 unvaccinated kids got sick, right (0.12 * 156)? Result 3 says 0.5% of the 595 healthy kids were unvaccinated, meaning 3 unvaccinated kids stayed healthy. 19+3 = 22 unvaccinated kids. It’s a super small sample but it implies an infection rate of 19/22 = 86%. 137 (that is, 156 – 19) vaccinated kids got sick, and 592 (595 – 3) vaccinated kids did not. That’s an infection rate of 19%. That would imply that refusing the vaccine would make you (.86/.19 =) 460% more likely to catch pertussis… not 2280%. Where does the 2280% come from?

    • Hi Jen, sorry for the delay in responding to this interesting question. The odds ratio that led to that 2,000 number is not computed the way you suggested because the “infection rates” are actually unknown. Yes, 19 unvaccinated kids got sick, but we don’t know the size of the population of unvaccinated kids that these came from, which would allow us to compute the rate of infection. We only know the % of non-infected kids that are unvaccinated. Adding these two is not the total sample of unvaccinated kids that would allow us to calculate an infection rate. In this case, the odds tells us the relative change in the probability that a unvaccinated kid will be in the infected group compared to a vaccinated kid. In this case, the odds ratio was 22.8 (Table 3 page 1449 of the original article). That is, compared to a vaccinated kid, unvaccinated kids were about 22 times more likely to be in the infected group. Hope this makes sense. Thanks, Nestor.

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