Researchers say they found no association between MMR vaccination and greater risk of autism, regardless of whether children had an older sibling with the condition.
In 1998, a study published in The Lancet - conducted by British researcher Andrew Wakefield and colleagues - suggested a link between the MMR (measles, mumps and rubella) vaccine and autism spectrum disorder (ASD).
The journal later retracted the study after it was deemed "fraudulent," and other studies have failed to reproduce its findings. But despite this, many parents continue to have fears about the safety of the MMR vaccine.
"Surveys of parents who have children with ASD suggest that many believe the MMR vaccine was a contributing cause," note the researchers of this latest study, including Dr. Anjali Jain, of the The Lewin Group, Falls Church, VA.
"This belief, combined with knowing that younger siblings of children with ASD are already at higher genetic risk for ASD compared with the general population, might prompt these parents to avoid vaccinating their younger children," they continue.
Many health care professionals say such beliefs have already led to undervaccination, and they have been cited as contributors to the recent measles endemic in the US.
"Lower vaccination levels threaten public health by reducing both individual and herd immunity and have been associated with several recent outbreaks of measles," note the authors, "with most cases occurring among unvaccinated individuals."
No harmful link between MMR vaccine and ASD, regardless of risk status
For their study, Dr. Jain and colleagues used a US administrative claims database linked to a large commercial health plan to assess MMR vaccine status and ASD occurrence among 95,727 children with older siblings. Of these, 1,929 had an older brother or sister with ASD.
During 2001-12, all children were continuously enrolled in the health plan from birth until a minimum of 5 years old. During 1997-2012, the children's older siblings were continuously enrolled in the health plan for at least 6 months.
During follow-up, 994 (1.04%) children were diagnosed with ASD. Of these cases, 134 (6.9%) occurred among children who had an older sibling with the disorder, while 860 (0.9%) of cases were diagnosed among children without an older sibling with ASD.
Among children without an older sibling with ASD, 78,564 (84%) had received at least one dose of the MMR vaccine at the age of 2 years, while 86,063 (92%) received the MMR vaccine aged 5 years.
MMR vaccination rates were lower among children with an older sibling affected by ASD, however; 1,409 (73%) were vaccinated at the age of 2 years and 1,660 (86%) were vaccinated aged 5 years.
Using the Cox proportional hazards model to assess ASD risk among vaccinated children, the researchers say they found no association between MMR vaccination - at one or two doses - and increased risk of ASD.
"We also found no evidence that receipt of either one or two doses of MMR vaccination was associated with an increased risk of ASD among children who had older siblings with ASD," say the researchers. "As the prevalence of diagnosed ASD increases, so does the number of children who have siblings diagnosed with ASD, a group of children who are particularly important as they were undervaccinated in our observations as well as in previous reports."
In conclusion to their analysis, the team says:
"These findings indicate no harmful association between MMR vaccine receipt and ASD even among children already at higher risk for ASD."
Study adds to growing evidence that MMR vaccine does not increase ASD risk
In an editorial linked to the study, Dr. Brian H. King, of the University of Washington and Seattle Children's Hospital, says the findings from Dr. Jain and colleagues add to the increasing evidence that MMR vaccination is not linked to greater risk of ASD.
"Although the evidence is already abundant that no relationship exists in the general population between MMR vaccine receipt and ASD risk, immunization rates remain low in certain populations and countries because of this inappropriate belief," Dr. King notes.
"Jain and colleagues evaluated two questions in their large insurance claims database," he continues. "Does the incidence of ASD differ in younger siblings of affected children who are immunized with MMR versus those who are not? And, for the population as a whole, does the incidence of ASD vary as a function of MMR immunization status? The answer to both questions is no."
"Taken together," adds Dr. King, "some dozen studies have now shown that the age of onset of ASD does not differ between vaccinated and unvaccinated children, the severity or course of ASD does not differ between vaccinated and unvaccinated children, and now the risk of ASD recurrence in families does not differ between vaccinated and unvaccinated children."
The study was funded by the National Institute of Mental Health, the National Institutes of Health and the US Department of Health and Human Services.
In February, a Spotlight from Medical News Today investigated whether ongoing concerns about vaccine safety are to blame for the measles epidemic in the US.