A study analysing the health records of 2.56 million children in the United States found no link between receiving the measles, mumps and rubella vaccine before the age of two and a later autism diagnosis. Published in The Pediatric Infectious Disease Journal, the research led by Todd L. Burstain of UW Medicine used a novel age-anchored landmark approach to compare vaccinated and unvaccinated children within defined age windows.
Study design and key findings
The researchers drew on Cosmos, a large electronic health record database spanning hospitals and clinics across the United States, and identified 2,560,035 children who had at least one routine primary care visit between 12 and 24 months.
The dataset recorded a childhood autism prevalence of 2.55 percent. The analysis focused on a specific diagnostic code for childhood autism, which is narrower than the broader autism spectrum, partly explaining why the prevalence in the sample was lower than the national estimate.
Children diagnosed with autism by 11 months were excluded to ensure the vaccine preceded the diagnosis. Follow-up extended to age eight or until December 2024. By 24 months, 91.4 percent of the children had received their first MMR dose.
The team grouped children into defined age windows, such as 11.5 to 12.5 months, 12.5 to 13.5 months, and up to 24 months. Within each window, they compared those who received the first dose during the window with those who remained unvaccinated at the window’s start.
To minimise bias, they adjusted for factors potentially related to both vaccination and autism: the mother’s age, race and ethnicity, gestational age at birth, whether the baby was small for gestational age or part of a multiple birth, diabetes or genital or urinary tract infections during pregnancy, neighbourhood disadvantage, and region. They avoided adjusting for post-vaccination factors to prevent distortion of the results.
Across all windows, the first MMR dose was not linked to an increased autism risk. In the primary analysis covering 11.5 to 24 months, the adjusted hazard ratio was 0.97, with a 99 percent confidence interval of 0.91 to 1.03. A hazard ratio of 0.97 translates to an estimated 3 percent lower rate of diagnosis among vaccinated children, a difference not statistically distinguishable from zero.
In two later windows, 14.5 to 17.5 months and 17.5 to 24 months, vaccinated children had slightly lower rates of autism diagnosis, with hazard ratios of 0.90 and 0.92. The authors attribute such small deviations to leftover bias in the data rather than any protective effect of the vaccine.
The findings held up under different analyses. Results were essentially unchanged with more birth-related factors added (hazard ratio 0.97) and without the neighbourhood measure, which was missing for 53 percent of children (hazard ratio 0.96). They also held after excluding children with only one recorded autism diagnosis.
To double-check their methodology, the researchers also tested a negative control exposure. They looked at the pneumococcal conjugate vaccine booster, administered around 12 to 15 months to prevent bacterial infections but with no theoretical link to autism. The similar result for the pneumococcal booster suggests the analysis was not generating spurious links tied to routine checkups, supporting the authors’ view that leftover bias is unlikely to explain the measles, mumps and rubella findings.
As with all research, there are a few things to keep in mind. The study relied on observational medical records, meaning the researchers could not control for every single variable in a child’s life.
Notably, the dataset did not include information on a family history of autism. If parents who already have an older child with autism are more hesitant to vaccinate their younger children, this missing detail could skew the data. Such a pattern would tend to make the vaccine look protective, a point the authors note alongside a 2015 study they cite which also found no link.
Additionally, the children who received the vaccine tended to have much longer follow-up periods in the medical system compared to the unvaccinated, with a median of 853 days for vaccinated children versus 366 for unvaccinated. Typically, spending more time interacting with healthcare providers creates more opportunities to receive a formal diagnosis, yet the researchers still found no link between the vaccine and autism.
The requirement that children have linked birth-parent records also excluded a substantial proportion of otherwise eligible children from the analysis. This means the findings might not capture the experiences of all children receiving medical care.
Future studies could incorporate external data on family history and socioeconomic factors to provide an even sharper picture. Scientists might also test if these findings hold true for broader autism spectrum diagnoses rather than the specific childhood autism diagnostic code used in this analysis.
The study, “Association Between First MMR Vaccination Before Age 2 Years and Childhood Autism in a U.S. EHR Cohort of 2.5 Million Children,” was authored by Todd L. Burstain, Dale Jacques, and Jennifer M. Burstain.
