A recent study in which researchers from Kaiser Permanente analyzed more than 196,000 children born into the health system between 2000 and 2010 revealed no link between an influenza infection during pregnancy and an increased risk of an autism spectrum disorder (ASD) diagnosis. However, the study did suggest that there might be an increased risk of ASD with maternal vaccination in the first trimester, although researchers noted that the suggestion was likely due to chance; it was not found to be statistically significant after adjusting for multiple comparisons.
“These findings do not call for changes in vaccine policy or practice, but do suggest the need for additional studies on maternal influenza vaccination and autism,” said Ousseny Zerbo, PhD, a researcher on the team and a member of the Autism Research Program at Kaiser Permanent Northern California.
The researchers examined records from 196,929 births delivered at Kaiser Premanente Northern California, starting at the 24th week of gestation. They then drew their data on maternal influenza infection and vaccination from the Kaiser Permanente Northern California inpatient and outpatient databases, and diagnosed “influenza infection” using positive laboratory test results as well as the International Classification of Diseases, Ninth Revision, Clinical Modification code. The team identified clinical diagnoses of ASDs using the same reference material. They considered an ASD diagnosis to constitute two or more records of ASD codes on a child’s medical history at any time from birth through June 2015.
The data review revealed that 1,400 mothers (0.7%) were diagnosed with influenza during their pregnancy during the study period. 45,231 mothers (23%) received a flu vaccine during pregnancy. A total of 3,101 children in the study were diagnosed with ASD (1.6%). “After adjusting for covariates, we found that maternal influenza infection or influenza vaccination anytime during pregnancy was not associated with increased ASD risk,” the team reported. The suggestion of an association between an increased risk of ASD and the administration of an influenza vaccine during the first-trimester could be explained by the numerous hypotheses tested in the study, which were a total of eight.
“This large study is reassuring for expectant mothers,” emphasized Dr. Zerbo. “The results of this study confirm that neither getting influenza during pregnancy or getting a flu vaccine during the second and third trimester are associated with any risk of autism in the child.” He added that the health risks associated with pregnant women or infants contracting the flu are known to be serious, so the team recommends that expectant mothers continue to receive the flu vaccine in consultation with their physician. “We would not advise a pregnant woman to wait until her second or third trimester to receive a flu shot,” Dr. Zerbo said, citing known risks for pregnant women who contract influenza and the study findings that ruled out increased risk of ASD “after adjusting for multiple comparisons.”
According to the Centers for Disease Control and Prevention (CDC), one vaccine ingredient, thimerosal—a mercury-based preservative used to prevent contamination of multi-dose vials of vaccines—has been studied specifically for associations with ASD and is still present in flu vaccines. However, it has been removed or reduced to trace amounts in all childhood vaccines despite conclusions suggesting that it does not cause ASD. “This [removal] was done as part of a broader national effort to reduce all types of mercury exposure in children,” a CDC spokesperson noted.