This one is by request from a colleague’s spouse. Apparently, they are in a text group that is asking about vaccines affecting intelligence, or are unvaccinated kids more intelligent. If you have a request for a topic (no personal medical advice), please contact me through my website.
It turns out that vaccinated children score higher on cognitive tests, shocking absolutely no one who understands that preventing brain-swelling diseases like measles is generally a solid strategy for academic success. Of course, critics love to treat this as a classic chicken and the egg argument, smugly pointing out that maybe it’s just that more intelligent, highly educated parents are the ones choosing to vaccinate in the first place, thereby passing down their superior IQs to their children. But whether it’s a case of smart parents making smart medical choices or vaccines actively safeguarding a child’s grey matter from preventable damage, the data remains stubbornly unbothered by the debate: the vaccinated kids are still the ones taking home the higher scores.
If you’ve spent more than seven minutes on parenting Instagram, you’ve absorbed the theory: somewhere between the two-month well-check and the kindergarten booster, a shadowy cabal of pediatricians is siphoning IQ points out of children and, presumably, selling them on Etsy.
It’s a compelling narrative. It has villains, syringes, and just enough Latin (“neurodevelopmental sequelae”) to sound like something you shouldn’t Google after 10 pm. There is, however, one small hiccup: researchers have actually looked. Repeatedly. In multiple countries. Over decades. And the results are, frankly, rude to the theory.
The inconvenient part where scientists checked.
Let’s start in the Philippines, where economists, people who are professionally suspicious of everything, including lunch, followed children from the Cebu Longitudinal Health and Nutrition Survey to see what happened to their brains years after their shots. The finding: kids who got the full set of routine childhood vaccines (measles, polio, TB, DPT) scored about half a standard deviation higher on cognitive tests than matched unvaccinated peers. Half a standard deviation. That is not a rounding error. That is the difference between “average” and “the kid who corrects the teacher’s grammar”.
If you were writing a screenplay called “Vaccines Steal Your Child’s Mind,” this is the part where your third act collapses.
Meanwhile, in the “but what about MMR specifically” department.
Ah, MMR, the vaccine equivalent of a celebrity that can’t go anywhere without paparazzi and conspiracy theories. So Polish researchers ran a prospective cohort study specifically on MMR and cognition. They gave kids actual, validated intelligence testing. They compared MMR vaccinated to MMR unvaccinated toddlers. The result? No significant difference in cognitive or intelligence test scores. None. Zip. Not even the artisanal, small-batch kind of difference. Which is boring. Which is why you’ve never seen it on a reel with dramatic string music.
Thimerosal: the ghost that keeps getting exorcised.
For the “but the mercury” crowd, a demographic that has never once looked at a tuna sandwich with the same energy, the NEJM did the definitive follow-up. Researchers tracked over a thousand children exposed to thimerosal-containing vaccines and tested them across 42 neuropsychological outcomes at ages 7 to 10. Forty-two. That is not a study, that is a psychological triathlon.
The conclusion: the weight of the evidence does not support a causal association between early thimerosal exposure and later neuropsychological deficits. A few scattered associations popped up, amusingly, in the direction of better performance in vaccinated kids, which is exactly what you’d expect from running 42 comparisons and letting chance do its little dance.
So what’s the pattern?
Zoom out and it’s almost comically consistent. Whether it’s economists in the Philippines, epidemiologists in Poland, or a very tired biostatistician in a New England Journal of Medicine sub-basement, the answer keeps being the same: vaccinated kids do at least as well as unvaccinated kids on cognitive testing, and in several long-term studies, they do meaningfully better, plausibly because they didn’t spend a chunk of early childhood febrile, hospitalized, or recovering from measles encephalitis, which, fun fact, is genuinely bad for a developing brain. Turns out not getting a preventable brain infection is good for your brain. Nobel committee, please form an orderly line.
The takeaway for parents who actually have to make this decision.
If you’re weighing the shot schedule against a viral post from someone whose credentials are “mother of three and blessed,” here’s the sober version: the best long-term data we have, cognitive testing, school achievement, IQ, neuropsychological batteries, shows that routine childhood vaccination is either neutral or beneficial for later intelligence. The “vaccines make kids dumber” hypothesis has been tested, retested, and dragged through more cohorts than a reality show. It keeps losing.
Your pediatrician is not stealing your kid’s IQ. Your pediatrician is, at most, running behind schedule and hoping the next patient isn’t the one with the four-page printout.
Any advice or information given in this blog should not supersede consultation with your pediatrician or practitioner of choice. Every child is unique, and medical decisions must be made in partnership with the healthcare provider who knows your child’s full history. This blog is intended for educational and informational purposes only, not as a substitute for personalized medical care.If you are looking for medical advice that is evidence-based, ethically grounded, and uncompromised by industry influence, you are in the right place.
Sources
Bloom DE, Canning D, Shenoy ES. The effect of vaccination on children’s physical and cognitive development in the Philippines. Applied Economics. 2012.
Mrożek-Budzyn D, Majewska R, Kiełtyka A. Measles, mumps and rubella (MMR) vaccination has no effect on cognitive development in children — results of the Polish prospective cohort study. PMC3684783
Thompson WW, Price C, Goodson B, et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med. 2007;357:1281-1292.