Danish study finds no association between MMR vaccine and autism, even within high-risk subgroups

1. In a large Danish cohort study, the measles-mumps-rubella (MMR) vaccine was not significantly associated with autism.

2. There was still no increased risk of autism with vaccination in subgroups of children with various risk factors for autism.

Evidence Rating Level: 2 (Good)

Study Rundown: In 1998, a controversial in the Lancet reported an association between childhood vaccine and autism.  This study was quickly retracted and many studies since have failed to find an association, but it has contributed to an enduring public mistrust of vaccination that the World Health Organization now believes to be one of the top ten threats to global health (WHO, 2019).  In this paper, a Danish team again found no association between the measles, mumps, and rubella (MMR) vaccine and autism.  However, their results are unique in that they also separately validated this conclusion for several subpopulations of children believed to be at increased risk for autism, including those with a sibling history of autism or an elevated autism disease risk score.  Likewise, there was no increased risk among children who had received other childhood vaccinations.  However, the study was biased by its reliance upon date of autism diagnosis as a metric for incidence.  If a child was vaccinated between the onset of symptoms and the date of diagnosis, the case could have been falsely linked to the vaccine when the disease process was already underway.  It is also possible that children with early symptoms of autism may have been less likely to be vaccinated.

Click to read the article in Annals of Internal Medicine

Click to read an accompanying editorial in Annals of Internal Medicine

Relevant Reading: Parent perspectives on childhood vaccination: How to deal with vaccine hesitancy and refusal?

In-Depth [retrospective cohort]: The study contained 657,461 children born to Danish mothers between 1999 and 2011.  Of these, 6417 children were diagnosed with autism and 6518 were censored or lost to follow up.  Around 95% of children received the MMR vaccine, and there were no appreciable trends in vaccination based on sex, autism risk score, or autism family history.  Standard risk factors for autism were confirmed, including an older father or mother, male sex, preterm birth, and smoking in pregnancy.  Following reasonable adjustment, the hazard ratio of autism in vaccinated vs unvaccinated children was 0.93 (95% CI, 0.85 to 1.02).  Similar values were found with different lengths of followup.  Importantly, the risk did not increase in children considered to be at high risk for autism or with autistic siblings.  It also did not increase with a second dose of the vaccine.

Image: PD

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