The largest single study to date has concluded that MMR vaccination does not increased autism risk, even among children with other risk factors for autism
A nationwide cohort study of all children born in Denmark to Danish born mothers between 1999 through 2010 has concluded the mumps, measles and rubella (MMR) vaccine does not increase the risk of autism.
It also found the MMR vaccine does not trigger autism in susceptible children, and is not associated with clustering of autism cases following vaccination.
The findings are published in Annals of Internal Medicine.
Researchers from the Statens Serum Institut in Denmark sourced data from the Danish population registry, capturing 5,025,754 person-years of follow-up from 1 January 2000 through to 31 August 2013.
Uptake of the MMR1 vaccine was 95.19% among the cohort, and there were no differences in vaccine uptake according to sex, birth cohort, autism risk score, or autism history in siblings.
Of the 657,461 children included in the analysis, 6,517 were diagnosed with autism.
Autism diagnoses included autistic disorder, atypical autism, Asperger syndrome, other pervasive developmental disorders, and unspecified pervasive developmental disorders.
Comparing children who received MMR vaccine with those who did not receive it, researchers calculated a fully adjusted autism hazard ratio of 0.93 (95% CI, 0.85 to 1.02) – meaning the MMR vaccine was not associated with an increased risk for autism.
Taking their study further, the researchers looked at subgroups of children, including those susceptible to developing autism, and those with environmental and familial autism risk factors.
They found the MMR vaccine does not trigger autism in susceptible children, for example those with a sibling history of autism.
It also did not lead to increased risk in children who received other childhood vaccines, or during certain time periods after receipt of the vaccine, and was not associated with clustering of autism cases after vaccination.
Meanwhile receipt of MMR vaccination reduced the risk for autism in girls (autism hazard ratio 0.79 [CI 0.64 to 0.97]) and in the 1999-2001 birth cohort (autism hazard ratio 0.84 [CI 0.73 to 0.96]).
The largest single risk factors for autism were an older or unknown father, an older mother, poor Apgar score, low birthweight, preterm birth, large head, assisted birth, and smoking in pregnancy.
Highest risk for autism was conferred by being a boy (hazard ratio 4.02 [CI 3.78 to 4.28]) and having siblings with autism at study entry (hazard ratio 7.32 [CI 5.29 to 10.12]).
These results add to the existing dozen or more studies that have shown the MMR vaccine is not linked with autism.
In the wake of the study results, experts encouraged parents and the public to seek advice about immunisation from reliable sources and qualified health professionals.
However they also expressed disappointment at the continual efforts that continue to be directed at vaccine safety research in response to hypotheses propagated by vaccine sceptics.
“Although it is fantastic to see another high-quality study refute the myth of an autism and MMR vaccine link, it is disappointing that substantial research efforts, time and funds have to continue to be directed toward disproving something that we already know to be incorrect; rather than investigating more accurate causes of autism,” said Dr Hannah Kirk, NHMRC Early Career Research Fellow at the Monash Institute of Cognitive and Clinical Neurosciences, and a Doctor in Developmental Neuroscience in the School of Psychological Sciences, at Monash University.
The study was funded by the Danish Ministry of Health and the Novo Nordisk Foundation, which is associated with a pharmaceutical company that does not manufacture the MMR vaccine.