Does the MMR (measles, mumps, and rubella) vaccine cause autism?

The number of autism cases has increased significantly in the past few decades, from approximately 1 in 2,500 children in 1966 to 1 in 68 in 2016. The increase in the number of autism cases is attributed in part to improved screening and broadening of the diagnostic criteria.

The alleged association between the MMR (measles, mumps and rubella) vaccine and autism began in 1998, when Andrew Wakefield and several coauthors published a research paper in The Lancet, a leading British medical journal, suggesting a link between them. Wakefield's findings caused great concern among parents in the UK and the US and led to a significant drop in vaccinations of children. Numerous subsequent studies have failed to support an association between the administration of the vaccine and autism spectrum disorder. Beginning in 2004, British investigative journalist Brian Deer wrote several articles accusing Wakefield of concealing conflicts of interest, manipulating evidence, and other unethical research practices. In 2010 The Lancet retracted Wakefield's study after several elements in the study were found to be "incorrect, contrary to the findings of an earlier investigation."

Another study (from 2003) reported elevated levels of measles antibodies in children with autism, which could indicate a link to the vaccine. However, four subsequent studies failed to find such a correlation. This inconsistency seems to indicate that the first study had some methodological error.

These two studies were the only studies to suggest a link between the vaccine and autism, while many studies by different organizations consistently fail to find such a relation.

When considering all the studies together, along with evidence such as an increase in autism cases in Japan following the termination of MMR vaccinations, the analysis concludes that it is extremely unlikely that the vaccine causes autism.

Another explanation for the evidence is that there is a broad, international conspiracy to cover up a link between the vaccine and autism (the “Conspiracy” hypothesis). A cover-up of this kind would require a massive conspiracy involving scientists, government agencies and pharmaceutical companies around the world. The analysis finds such a cover up to be very unlikely when compared to known past cover ups (like the conspiracy to hide the health risks associated with smoking), as it would involve a much larger number of organizations, many of which have little motivation to participate in such a conspiracy.

Note: The likelihood values that support the "Conspiracy" and "Not established" hypotheses have been estimated generously, since both are exceedingly unlikely hypotheses, yet considered by many to be plausible explanations. By inflating the estimates for the least likely hypotheses, we provide a wide margin of error and ensure that the conclusion is correct even if some evidence or data is missing from the analysis.

Note to Parents: This analysis measures the probability that the MMR vaccine can cause autism, even if rarely. The probability of 0.1% does not mean a 1 in 1000 chance of a child becoming autistic! It means that the evidence for the MMR vaccine causing autism is no different from the evidence for any other substance (say, apple sauce) causing autism. This conclusion is a strong indication that parents should vaccinate their children.