Tdap vaccination in pregnant women and autism risk

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Prenatal tetanus, diphtheria, acellular pertussis (Tdap) vaccination is not associated with an increased risk of autism spectrum disorder (ASD) in offspring, according to a study published in Pediatrics.

In response to rising incidence of pertussis (whooping cough), the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) recommended in October 2012 that all pregnant women, whether previously vaccinated or not, receive the Tdap vaccine between 27 and 36 weeks of gestation to prevent pertussis in their offspring. Pertussis is a highly contagious infection caused by Bordetella pertussisthat can lead to hospitalization and death in young infants. When given during pregnancy to mothers, the vaccine is 91.4% effective in offering some immunity to infants until they reach two months of age and are due to receive their first diphtheria-tetanus-acellular pertussis vaccine.

Previously, it had been found that Tdap vaccine does not increase the risk for a variety of maternal and pediatric outcomes, including small-for-gestational-age size, preterm delivery, or low birth weight. As there has been great speculation and concern among consumers about the potential of vaccines to cause ASD and limited data on vaccines given during pregnancy and autism risk, Kaiser Permanente Southern California Department of Research & Evaluation undertook the first-ever study examining any association between the two. 

The prenatal period is “strongly implicated” in the etiology of ASD, said the authors, who conducted a retrospective cohort study of mothers who delivered babies between January 1, 2011 and December 31, 2014 at Southern California hospitals within the Kaiser Permanente system. The population was restricted to pregnant women who conceived naturally and gave birth to live singleton infants at 22 to 45 weeks of gestation. The study included 81,993 children. In the 2012 birth cohort, 26% (5,407 of 20,553) of the children had mothers who had been vaccinated, whereas 79% (16,882 of 21,433) had vaccinated mothers in 2014. Most mothers were vaccinated at 28 to 29 weeks of pregnancy. The cohort was followed for 1.2 to 6.5 years. 

Compared to unvaccinated women, vaccinated women were more likely to be:

• of Asian American or Pacific Islander descent

• Well-educated

• Nulliparous

They were also more likely to have received the influenza vaccine prenatally and to have given birth at term, defined as >37 weeks of gestation. 

The incidence of ASD in the vaccinated group was 3.78 per 1,000 person-years versus 4.05 per 1,000 person-years in the unvaccinated group (unadjusted HR=0.98, 95% CI:0.88-1.09; adjusted HR=0.85, 95% CI=0.77-0.95). Most of the children with autism were diagnosed at 2 to 4 years of age. A decline in autism rates was noted from 1.8% to 1.2% in the vaccinated group and from 2.0% to 1.5% in the unvaccinated group across the birth years studied. The general autism rate in the United States was 1.7% of children. 

The results were consistent across all birth years included in the study and among first-born children. 

The authors suggested that more studies be conducted looking at additional birth years and with longer follow-up, but confirmed that prenatal Tdap vaccination is safe and desirable to protect vulnerable newborns from potentially deadly pertussis infection.