Minneapolis, MN—For the last 4 years, the tetanus, diphtheria, and acellular pertussis (Tdap) vaccine has been recommended by the CDC’s Advisory Committee on Immunization Practices for pregnant women.

Concerns have been raised about the safety of the vaccine for fetuses, however, because of an increase in microcephaly in Brazil in 2015, shortly after the initiation of a maternal Tdap program in that country. Public-health officials have suggested that the increase was more likely associated with Zika virus infections, which were occurring at the same time.

A large new study sought to better pinpoint the cause of the problem, especially since previous small observational studies reported no increased risks for birth defects following maternal Tdap vaccination, although none focused on microcephaly.

To do that, a study team led by researchers from HealthPartners Institute in Minneapolis conducted an analysis that included more than 300,000 births. Results, published as a research letter recently in the Journal of the American Medical Association, indicate that Tdap administration during pregnancy is not significantly associated with increased risk for microcephaly or for structural birth defects in offspring.

Included in the study was data from live births at seven Vaccine Safety Datalink sites—Northern California, Southern California, Colorado, Minnesota, Oregon, Washington, and Wisconsin—from January 2007 through September 2013. Using diagnostic codes assigned during medical visits during the first year of life, researchers compared prevalence of structural birth defects between infants born to women who received Tdap during pregnancy and unvaccinated women.

Analyses of maternal Tdap vaccination from 27 to 36 weeks’ gestation were limited to 2010-2013 for California sites and to 2012-2013 for other sites. 

Maternal Tdap administration was not significantly associated with increased risk for microcephaly for vaccinations occurring at any week of pregnancy.

“These results expand upon what is known about maternal Tdap vaccination safety to include information about structural birth defects and microcephaly in offspring,” the researchers explain. “The findings are potentially limited by incomplete data on Tdap vaccinations (making it possible to misclassify women’s immunization status), diagnosed structural birth defects, and important covariates (including maternal alcohol use), as well as inability to study birth defects resulting in pregnancy loss or elective termination. The findings support recommendations for routine Tdap administration during pregnancy.”
 
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