Do vaccinations cause autism? What the studies show

A retracted paper from 1998

In 2010, The Lancet published a retraction of a 1998 paper on bowel problems and developmental disorders in children. ^[3] This article says no more about the content or the reasons for the retraction, because the wording of the retraction notice was not evaluated.

What the 2014 meta-analysis shows

Taylor and colleagues searched several databases up to April 2014 for studies that examined vaccinations and the later development of autism or autism spectrum disorder. They included five cohort studies with a total of 1,256,407 children and five case-control studies with 9,920 children. ^[1]

• The cohort studies showed no link between vaccination and autism (odds ratio 0.99; 95 % confidence interval 0.92 to 1.06) or autism spectrum disorder (0.91; 0.68 to 1.20). For MMR (0.84; 0.70 to 1.01), thiomersal (1.00; 0.77 to 1.31) and mercury (1.00; 0.93 to 1.07) too, no link was found. ^[1]
• The case-control studies found no indication of an increased risk after MMR, mercury or thiomersal. ^[1]
• The authors concluded that vaccinations are not linked to the development of autism and that neither vaccine components (thiomersal, mercury) nor several vaccinations at once (MMR) are related to it. ^[1]

An odds ratio close to 1 means: vaccinated and unvaccinated children practically do not differ in the risk of autism.

What the 2019 Danish study shows

Hviid and colleagues evaluated register data on 657,461 children born in Denmark between 1999 and 2010. During the observation period, 6,517 children were diagnosed with autism. Comparing MMR-vaccinated with non-MMR-vaccinated children, after accounting for age, birth year, sex, other vaccinations, autism in siblings and risk factors, a hazard ratio of 0.93 resulted (95 % confidence interval 0.85 to 1.02). ^[2] In subgroups too (for example children with siblings with autism or with risk factors) and in certain periods after vaccination, no increased risk was found. The authors regarded this as strong support for the view that MMR does not increase the risk of autism, does not trigger autism in susceptible children and that cases do not cluster after vaccination. ^[2]

Limits of the studies

• Observational studies. Both papers evaluate observed data; they are not experiments. That is precisely why large amounts of data and accounting for other influences are important. ^{[1; 2]}
• Register data. In the Danish study, no individual patient records were checked; the authors expressly name this as a limitation. ^[2]
• Parts read. For this article, the summaries (abstracts) of the studies were evaluated, not the full texts. Some details of the methods are therefore not presented here.
• Not answered: This article says nothing about other questions on vaccinations, such as recommendations on the vaccination schedule or on individual vaccines.

What else you should know

• Questions about vaccination belong in the paediatric practice. There you can discuss the vaccination schedule and your questions.
• Where autism comes from: on causes see Causes of autism.
• Recognising myths: how to put dubious promises in context is described here: Recognising dubious offers.

Sources and literature

1. Taylor LE, Swerdfeger AL, Eslick GD. Vaccines are not associated with autism: an evidence-based meta-analysis of case-control and cohort studies. Vaccine 2014;32(29):3623–3629. https://doi.org/10.1016/j.vaccine.2014.04.085 – Meta-analysis
2. Hviid A, Hansen JV, Frisch M, Melbye M. Measles, mumps, rubella vaccination and autism: a nationwide cohort study. Annals of Internal Medicine 2019;170(8):513–520. https://doi.org/10.7326/M18-2101 – Cohort study
3. Retraction – Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. The Lancet 2010;375(9713):445. https://doi.org/10.1016/S0140-6736(10)60175-4 – Retraction notice