New research has pulled together more than a dozen studies on measles and pertussis outbreaks and offers further confirmation that lower vaccination rates contribute to these outbreaks. The study, appearing in JAMA, is a meta-analysis that involved examining 18 different studies’ results before drawing conclusions. One of those conclusions, as we discuss in our book, is that weaknesses in the pertussis vaccine have been a major cause of pertussis outbreaks even as vaccine refusal has exacerbated the problem.
The other major conclusion, as Tara discusses in depth at Forbes, is that people who were intentionally unvaccinated against measles have played the largest role in outbreaks of that disease, the most contagious one humans are currently susceptible to. Yet the findings of the study have also led to some misconceptions about the effectiveness of the measles vaccine. The study found that just over half all the measles cases involved individuals who were not vaccinated. Some readers have taken that to mean the vaccine is only 50% effective if the other half were vaccinated. Doesn’t that show the vaccine doesn’t work very well?
That conclusion that the measles vaccine doesn’t work is an excellent example of the base rate fallacy — it ignores how many exposed people were vaccinated and how many exposed people were not vaccinated. There have always been outbreaks in which the number of infected vaccinated individuals exceeds the number of infected unvaccinated individuals. That happens because the percentages of vaccinated and unvaccinated cases differ.
A hypothetical scenario can help clarify the confusion: Let’s say 9,600 in a community of 10,000 people are vaccinated. An outbreak occurs. Let’s say 200 unvaccinated people get sick and 1,000 vaccinated people get sick. That means five times as many people who were vaccinated got the measles than weren’t vaccinated, right? But wait — 1,000 out of 9,600 vaccinated people is 10%. And 200 out of 400 unvaccinated people is 50%. Just one out of every ten vaccinated people got sick while half of those who were unvaccinated got sick. So in this hypothetical example, the risk for the unvaccinated was actually five times greater. While these numbers are made up for the sake of providing a simple illustration, this phenomenon is exactly what’s occurring in these outbreaks. In fact, in one study, the researchers determined that unvaccinated children were a whopping 35 times more likely to catch measles than vaccinated children.