Research & evidence
A study of vaccine introductions, withdrawals and changes in uptake across 40 countries, set alongside the wider evidence.
Research paper29 October 2026WHO evidence review ↗
01 / Results from the paper
The analysis compares recorded autism with each jurisdiction’s preceding trend, around 404 schedule changes in 40 countries.
Repeated comparisons are combined within countries, then country summaries are pooled.
The pooled estimate
The model translates population changes using the change in vaccination uptake. A ratio of 1 means no change on this scale.
The vertical line marks 1: no change.
The estimate is close to 1, the no-change value. The two intervals show uncertainty with and without a shared-bias allowance.
| Cohort | Uptake-scaled model ratio | 95% random-effects interval | Countries | Comparisons |
|---|---|---|---|---|
| −6 | 0.976 | 0.869–1.096 | 37 | 28,031 |
| −5 | 0.985 | 0.876–1.109 | 36 | 29,716 |
| −4 | 1.010 | 0.903–1.130 | 38 | 31,017 |
| −3 | 1.021 | 0.912–1.143 | 38 | 32,682 |
| −2 | 1.016 | 0.910–1.134 | 39 | 33,445 |
| −1 | 0.994 | 0.887–1.114 | 36 | 34,716 |
| 0 | 0.965 | 0.854–1.090 | 35 | 31,724 |
| +1 | 0.967 | 0.863–1.082 | 38 | 32,516 |
| +2 | 0.994 | 0.891–1.108 | 39 | 32,329 |
| +3 | 0.977 | 0.874–1.091 | 38 | 32,307 |
| +4 | 0.981 | 0.879–1.095 | 37 | 33,662 |
| +5 | 0.989 | 0.885–1.106 | 36 | 32,819 |
| +6 | 0.972 | 0.870–1.087 | 37 | 31,979 |
Complementary analyses examine control outcomes, vaccine-and-place contrasts, vaccine pairs and local coverage.
For ratios, 1 means no association; 1.25 means 25% higher on the stated scale. The cohort-slope row reports percent change per birth cohort, with zero meaning no change.
| Analysis | Scope | Estimate | 95% interval | Measure and interval type |
|---|---|---|---|---|
| Schedule changes: primary model | 40 countries; 39,753 comparisons | 0.973 | 0.836 to 1.131 | Uptake-scaled ratio; random-effects interval |
| Autism compared with control outcomes | 25 countries; 49,966 comparisons | 0.913 | 0.759 to 1.098 | Autism-minus-control model ratio; random-effects interval |
| Additional cohort-slope analysis | 39 countries; 35,365 comparisons | -1.73% | -8.05% to +5.02% | Percent change per cohort; placebo-calibrated interval |
| Shared mean across 23 contexts | 23 contexts | 0.937 | 0.599 to 1.458 | Shared mean ratio; generalized-profile credible interval |
| Shared mean across 17 contexts | 17 contexts; 8,538 cells | 0.987 | 0.638 to 1.519 | Shared mean ratio; credible interval |
| Vaccine-pair interactions | 27 vaccine-pair contrasts | 0.967 | 0.881 to 1.063 | Interaction multiplier; credible interval |
| Kindergarten MMR coverage | 14 states | 1.002 | 0.950 to 1.057 | Autism share of special education per 10 coverage points; credible interval |
| Early-childhood disability records | 6 states | 1.002 | 0.761 to 1.321 | Autism share of disability classifications per 10 coverage points; modified Hartung-Knapp interval |
Each row answers its stated question. A context is a vaccine or schedule contrast in a particular setting; some observations are shared across rows. The primary model’s shared-bias interval is shown above.
The matched analysis compares autism with control outcomes in the same reporting systems. Context models summarize vaccine-and-place effects; the 17- and 23-context models cover different sets of comparisons. The interaction multiplier measures departure from the product of separate effects. Local MMR rows measure autism’s share of disability classifications. These rows are reported separately rather than pooled together.
Random-effects intervals allow country results to vary. Credible intervals describe uncertainty under the Bayesian model. Placebo calibration uses fake event dates. Modified Hartung–Knapp intervals adjust uncertainty when combining a small number of states. The cohort-slope analysis describes differences across successive birth cohorts, rather than accumulation within individuals.
02 / Robustness
The pooled estimate stays close to 1 with alternative trends, shorter follow-up, measured-uptake samples and country omissions.
| Countries included | Countries | Estimate | 95% interval |
|---|---|---|---|
| All countries | 40 | 0.973 | 0.836–1.131 |
| Exclude United States | 39 | 0.977 | 0.836–1.141 |
| Exclude Sweden | 39 | 0.969 | 0.830–1.131 |
| Exclude United States and Sweden | 38 | 0.974 | 0.831–1.141 |
Each row uses the same country-level model. The intervals allow country effects to vary and exclude the separate shared-bias allowance. All include 1, the no-change value.
Removing these countries has little effect on the pooled estimate. Country exclusions assess influence; they do not test every source of bias. Download these results (CSV) ↗
| Model or sample | Ratio | 95% random-effects interval | Countries | Overlapping comparisons |
|---|---|---|---|---|
| Linear trend: primary model | 0.973 | 0.836 to 1.131 | 40 | 39,753 |
| Quadratic trend | 0.960 | 0.828 to 1.112 | 39 | 34,775 |
| Spline trend | 0.980 | 0.848 to 1.133 | 37 | 28,296 |
| End follow-up four cohorts after the change | 0.963 | 0.829 to 1.118 | 39 | 36,108 |
| Measured vaccination uptake | 0.934 | 0.717 to 1.217 | 14 | 8,355 |
| Modeled vaccination uptake | 0.967 | 0.807 to 1.159 | 37 | 15,055 |
| Mixed measured and modeled uptake | 0.963 | 0.779 to 1.190 | 16 | 16,343 |
| Large increases in uptake | 0.984 | 0.843 to 1.148 | 38 | 20,514 |
| Countries with subnational data | 0.993 | 0.818 to 1.205 | 18 | 39,532 |
| Exclude vaccine mandates | 0.970 | 0.834 to 1.129 | 40 | 37,386 |
Every country-omission interval includes 1. Each table row uses its own eligible sample; the trend comparisons therefore change both the adjustment and the eligible observations. Countries can appear in more than one uptake group. These intervals exclude the separate shared-bias allowance.
The estimates concern recorded autism. Their interpretation depends on uptake measurement, event timing and the assumed bias model; source-specific limitations are detailed in the technical companion.
03 / How the study works
Follow the same birth cohorts from vaccination to later outcome records. Compare the affected cohorts with their expected path, then combine country summaries.
Align vaccination early in life with autism records collected later for those same birth cohorts.
Compare at the same age, or within comparable age bands where the source allows. Counts from health, school and service systems each measure a different form of recorded autism.
A rising line alone cannot identify an effect. The comparison asks whether affected cohorts depart from the path expected without the exposure change.
Fewer children vaccinated → more vaccinated
With the same effect per vaccinated child and outcomes compared at the same age, an abrupt increase in uptake predicts an abrupt difference between birth cohorts. An effect that develops within a person does not, by itself, turn that cohort step into a ramp.
The horizontal axis is birth cohort; outcomes are measured later, at comparable ages. Calendar-year totals can smooth a cohort step by mixing cohorts. Recording changes are examined separately in the diagnostic checks.
Combine repeated analyses within each country, then pool the country summaries.
The assumed random-effects model allows countries to differ.
04 / The wider evidence
Follow individual children. Examine specific vaccines. Look at whole populations. Each approach tests a different part of the claim.
Large cohort studies compare autism diagnoses in vaccinated and unvaccinated children. WHO’s review finds that the strongest studies do not support a causal link.
The research reviewed by WHO ↗MMR, thimerosal and aluminium have each been investigated. The evidence does not support them as causes of autism.
WHO’s autism overview ↗Schedule-change studies compare affected birth cohorts with their expected path. The findings above add this population-level test to the wider evidence.
Back to the study results ↑05 / Common questions
Autistic characteristics may become apparent in early childhood, when children also receive routine vaccines. An event occurring first does not establish that it caused what followed. Studies need a comparison that tests that explanation. About autism and early development ↗
Research has examined MMR, thimerosal and aluminium. WHO’s review concludes that the available high-quality evidence indicates that vaccines, including those containing thimerosal or aluminium, do not cause autism. Read the WHO assessment ↗
These data count children identified by health, education or service systems. The manuscript distinguishes changes in identification and reporting from changes in underlying incidence; a rise in recorded diagnoses alone cannot identify its cause. The paper’s discussion of diagnosis ↗
Yes. Safety research identifies specific risks: for example, fever-related seizures occur more often after a first MMRV dose than after separate MMR and varicella injections at the same visit. An autism conclusion does not replace the evidence about other outcomes. CDC: MMRV safety studies ↗
Three analyses address different questions: vaccine-pair interactions, total scheduled load, and whether background schedule modifies vaccine-specific effects. The average pairwise interaction is close to 1. The direct-load analysis uses up to 46,491 aggregate outcome cells across seven countries, with results and precision varying by country and exposure measure. It relies on reconstructed schedules and restrictions on age and calendar-time patterns. Schedule-load results and methods →
06 / Read further
Read the argument in full, examine the methods, or explore the underlying results and source records.
Findings, methods and interpretation.
29 October 2026 edition.
Detailed models and supporting analyses.
29 October 2026 edition.
Explore estimates by setting and vaccine, and examine the methods and source data.
Open the technical companion →