Prenatal Tdap Vaccine Not Linked to Autism, Study Finds

August 13, 2018

Women who receive the tetanus, diphtheria, and acellular pertussis (Tdap) vaccine during pregnancy are no more likely to have a child with an autism spectrum disorder (ASD) than women who forgo the vaccine, according to a study published in Pediatrics.

Rising rates of pertussis and waning pertussis immunity led the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices to recommend Tdap vaccinations for pregnant women. The prenatal vaccinations have been shown to be 91.4% effective at providing newborns with some immunity to pertussis during the first 2 months of life when they are too young to be vaccinated themselves. So, growing numbers of women have been opting for prenatal Tdap shots.

Previous studies have shown that Tdap is not associated with low birth weight or preterm deliveries. Now, Tracy A. Becerra-Culqui, PhD, MPH, a postdoctoral research fellow from the Department of Research and Evaluation at Kaiser Permanente Southern California, and colleagues provide more reassurance about the safety of prenatal Tdap vaccinations by showing it is not associated with an increased risk of the child developing an autism spectrum disorder.

"We provide evidence supporting the ACIP's recommendations to vaccinate pregnant women to protect vulnerable infants, who are at highest risk of hospitalization and death after pertussis infection," Becerra-Culqui and colleagues write.

The retrospective study looked at the electronic health records of almost 82,000 children born at Kaiser Permanente hospitals in southern California between January 1, 2011, and December 31, 2014, and their mothers. Of these children, 1341 (1.6%) were eventually diagnosed with an ASD. But the incidence of these disorders was not significantly different, with an ASD rate of 3.78 per 1000 children whose mothers had received Tdap prenatally and a rate of 4.05 per 1000 children with mothers who were not vaccinated during pregnancy.

In fact, the hazard ratio suggested the risk for ASD may actually be lower in children exposed to the prenatal Tdap vaccine (hazard ratio, 0.85, 95% confidence interval, 0.77 - 0.95). This might be explained by the fact that infections during pregnancy resulting in prolonged fevers are associated with elevated risk for ASD.

"Our results potentially indicate that the maternal Tdap vaccine affects immune trajectories protecting infants against infections that would otherwise lead to neurodevelopmental alterations," the authors write. However, they note, more research is needed to understand the potential underlying relationships.

The study relied on electronic health records to identify children with ASD. Children who were diagnosed with ASD outside the system might have been missed. However, the 1.6% overall prevalence of ASD in the study is comparable to the estimated ASD prevalence of 1.7% in US 8-year-olds, suggesting few children were likely missed. Children might also have been misdiagnosed as having autism, but the authors say this is unlikely because the Kaiser system was using mental health professionals for diagnoses, in accordance with a 2012 California law requiring insurers to cover the condition.

This study was funded by Kaiser Permanente of Southern California. Study authors Becerra-Culqui, Tseng, and Sy received funding from GlaxoSmithKline Biologicals for a previous study on the tetanus, diphtheria, acellular pertussis vaccine (Boostrix) during pregnancy; Getahun received research grant support from Bayer AG for unrelated studies; Tseng, Getahun, and Sy received research funding from the Centers for Disease Control and Prevention for the Vaccine Safety Datalink project. The remaining coauthor has disclosed no relevant financial relationships.

Pediatrics. Published online August 13, 2018. Abstract

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