A new World Health Organization systematic review has concluded that the best available evidence does not support a causal association between vaccines — including vaccines containing thiomersal — and autism spectrum disorder.
The review updates earlier WHO assessments by examining peer-reviewed research published between 2010 and August 2025 and evaluating study quality, strength of evidence and risk of bias.
What the review found
WHO reports that the most methodologically rigorous primary studies did not support a causal relationship between vaccination and autism spectrum disorder.
Studies that reported positive associations were judged to have very low strength of evidence and high risk of bias. WHO also reports that none of the included meta-analyses supported a causal association.
The review additionally considered other neurodevelopmental disorders and concluded that the overall evidence continues to support the safety of current vaccines with respect to autism and other neurodevelopmental outcomes.
Why study quality matters
Not all studies carry the same evidentiary weight.
A result can be distorted by small sample sizes, poor control of confounding factors, selective reporting, inappropriate comparison groups or study designs that cannot establish whether vaccination preceded and caused the observed outcome.
Systematic reviews are useful because they do more than count how many studies point in one direction. A rigorous review also asks whether the underlying studies were capable of answering the question reliably.
What thiomersal is
Thiomersal, also known as thimerosal, is an organomercury-containing preservative used in some multidose vaccine formulations.
Concerns about a possible relationship with autism have been examined repeatedly over the past two decades. WHO's Global Advisory Committee on Vaccine Safety reviewed the issue several times between 2002 and 2012 and found no evidence supporting a causal link.
The new review was commissioned to update that evidence base through 2025.
What this review does and does not establish
The review addresses whether the available scientific evidence supports a causal relationship between vaccination or thiomersal exposure and autism spectrum disorder.
Its conclusion is that the evidence does not support such a relationship.
That does not mean vaccines are incapable of causing any adverse effects. Like other medical interventions, vaccines can have recognised adverse reactions, which are monitored through clinical trials, pharmacovigilance systems and post-marketing surveillance.
The relevant question here is narrower: whether vaccines or thiomersal cause autism. WHO's updated evidence review concludes that they do not.
Why this matters
Public-health decisions are strongest when they distinguish persistent claims from evidence that has been repeatedly tested.
This review is useful because it evaluates a large modern evidence base using explicit quality criteria rather than relying on individual studies or anecdotes.
Primary source
- World Health Organization. Vaccines, thiomersal and autism spectrum disorder: evidence review 2010–2025. 3 September 2026. https://www.who.int/publications/i/item/vaccines--thiomersal-and-autism-spectrum-disorder--evidence-review-2010-2025