One statement people make a lot on the internet is “where’s a study of vaccinated vs. unvaccinated populations?” Well, here’s one: The effect of vaccination on children’s physical and cognitive development in the Philippines.
When comparisons between vaccinated and unvaccinated populations are proposed, we usually think of the U.S. and trying to work with the small unvaccinated population in a larger vaccinated population. Here we see the reverse: a smaller vaccinated population in a majority unvaccinated population.
What did they find? Here’s the abstract:
We use data from the Cebu Longitudinal Health and Nutrition Survey (CLHNS) in the Philippines to link vaccination in the first 2 years of life with later physical and cognitive development in children. We use propensity score matching to estimate the causal effect of vaccination on child development. We find no effect of vaccination on later height or
weight, but full childhood vaccination for measles, polio, Tuberculosis (TB), Diphtheria, Pertussis and Tetanus (DPT) significantly increases cognitive test scores relative to matched children who received no
vaccinations. The size of the effect is large, raising test scores, on average, by about half an SD.
That’s right. Test scores are increased in the vaccinated population. Higher. They did better.
The study highlights many of the difficulties in doing a vaccinated/unvaccinated population comparison: how to control for confounds. The population that choses the minority approach, be it vaccinating (in the Philippines) or not vaccinating (as in the U.S.) are likely different in other respects as well. Small sample sizes also a limitation. The authors acknoweldge this:
While our results are statistically significant, the sample size is relatively small due to the restriction of the sample to the common support. In addition, the matching of treatment and control groups may be imperfect if there are unobserved confounding factors that affect both vaccination and cognitive development. We therefore do not see our results as definitive. However, the results do however highlight the potential significance of vaccination as a human capital investment and suggest that further research in this area is warranted.
So, let’s consider this question: if there is a real correlation, is it the vaccination itself (unlikely in my opinion) or preventing the diseases (much more likely)? Since as I’ve indicated, I tend towards the latter explanation, let’s consider this: another effect of herd immunity might be cognitive. Since my family and the vast majority of families in the U.S. vaccinate, many diseases are not seen here. Even the unvaccinated are protected.
So, when Jenny McCarthy or others say, “I’d take measles any day over autism”, aside from making the huge mistake of assuming that autism and vaccination are linked, she may be saying “I’d take a half-standard-deviation drop in cognition over vaccination”.
I await the inevitable, “we asked for a comparison of vaccinated vs. unvaccinated populations, but not that comparison”.
By Matt Carey