True or False: There are more vaccinated pertussis cases than unvaccinated
True, but if you use this as a platform for your anti-vaccination beliefs, you are either intentionally misleading people or do not fully understand the data.
Hi reader. In the face of a massive pertussis outbreak in the state of Washington, antivaxxers are up to their old tricks of misusing information to spread an incorrect message. Of late, all over the anti-vax world I am seeing repeated assertions that “the rate” of pertussis is greater in vaccinated than unvaccinated people, and that this means that 1) vaccines don’t work or even 2) that vaccines cause pertussis. Therefore, the following post is my public service message. I know you were told there would be no math, but we are definitely going to need a little math for the post today.
Recently at this very blog, a poster provided this link as proof that “whooping cough outbreaks are HIGHER among vaccinated children”, and “The…link CLEARLY indicates that pertussis rates are higher in vaccinated children”. In the linked article, it states:
Of the 132 [pertussis] patients under age 18, 81 percent were up to date on recommended whooping cough shots and eight percent had never been vaccinated. The other 11 percent had received at least one shot, but not the complete series.
Proof that the poster is right, right? Wrong. Not even close. Here is the statistical explanation for why the poster is wrong, even though 81% of the pertussis cases were up-to-date on their boosters according to the guidelines at the time. In short, the issue is simple. No vaccine is perfect. And there are far more people who are vaccinated than who are not–for now, at least.
The article linked by the poster was in regard to the 2010 California pertussis outbreak, in which 10 infants died. At that time, approximately 93.1% of kindergarteners had received their pertussis vaccination. To be conservative, we will therefore assume that 93% of Californians under age 18 had been vaccinated for pertussis and that 7% had not. I will now use a hypothetical example with nice round numbers to illustrate the problem with stating that the rate of pertussis was higher in vaccinated vs. unvaccinated children.
Immanuel Pfeiffer Middle School has 1000 students enrolled. The students are vaccinated at the same rate as the rest of California (93%); therefore, 930 students are vaccinated and 70 are not. There is an outbreak of pertussis at IPMS, with 100 cases. Of these, 81% of the cases are vaccinated (n=81), and 19% are unvaccinated (n=19). The question, then, is what is the rate of pertussis in vaccinated vs. unvaccinated students?
The rate for vaccinated students is: 81/930=8.7%
The rate for unvaccinated students is 19/70=27.1%
It seems to be pretty clear that the rate of pertussis is far higher in unvaccinated students than in vaccinated students. After this simple explanation using real-world data, I fully expect anti-vaxxers to understand just how wrong they are when they use data regarding number of pertussis cases who have been vaccinated to erroneously assert that vaccines “don’t work”. Like anything else related to human beings, vaccines aren’t perfect…but they do substantially reduce the risk of disease. Boosters are necessary, and the more people who understand that, the greater the protection for the entire community.