A retrospective cohort study in more than 80,000 children has found no evidence of an increased risk of autism spectrum disorder associated with prenatal tetanus, diphtheria, and acellular pertussis (Tdap) immunization.
Piotr Marcinski/Thinkstock
Of 81,993 children born between 2011 and 2014, 1,341 children (1.6%) were diagnosed with autism spectrum disorder. The incidence of autism spectrum disorder was 3.78 per 1,000 person-years in the Tdap-vaccinated group, and 4.05 per 1,000 person years in the unvaccinated group, representing an unadjusted hazard ratio of 0.98 and an adjusted hazard ratio of 0.85. This was consistent across all the birth cohorts.
Prenatal immunization rates with the prenatal Tdap vaccine ranged from 26% of the 2012 birth cohort to 79% of the 2014 birth cohort, and mean gestational age at vaccination was 28 weeks.
Tracy A. Becerra-Culqui, PhD, MPH, and colleagues of the department of research and evaluation at Kaiser Permanente Southern California, Pasadena, said this was the first study to look at the risk of autism spectrum disorder after maternal exposure to the Tdap vaccine, to their knowledge. “Our results potentially indicate that the maternal Tdap vaccine affects immune trajectories protecting infants against infections that would otherwise lead to neurodevelopmental alterations.”
They highlighted several strengths of their study. One was that maternal Tdap vaccination and information on autism spectrum disorder both were derived from EHRs and therefore not subject to recall bias. The study, published online in Pediatrics, also included children diagnosed with autism spectrum disorder from age 1 year onwards, reflecting the latest evidence on screening and diagnosis of autism.
“Our weighting procedures enabled us to balance the Tdap-exposed and -unexposed groups to compare two populations that were comparable in important measured confounding factors,” Dr. Becerra-Culqui and associates noted.
The investigators found that women who received the Tdap vaccine during pregnancy were more likely to be Asian American or Pacific Islander, to have a bachelor’s degree or higher, be nulliparous, to have also been vaccinated prenatally against influenza, and to deliver at term, compared with unvaccinated women.
However the authors did note that their follow-up was limited to 6.5 years for the earliest birth cohort, and 3.5 years for the latest cohort, so they may not have picked up children who received a later diagnosis of autism spectrum disorder.
The study was supported by Kaiser Permanente Southern California. Five authors declared funding from GlaxoSmithKline, Bayer AG, or the Centers for Disease Control and Prevention for unrelated or separate studies.
SOURCE: Becerra-Culqui T et al. Pediatrics. 2018;142(3):e20180120.