Immunization with tetanus, diphtheria, and acellular pertussis (Tdap) vaccine during the third trimester of pregnancy is associated with higher concentrations of pertussis toxin antibodies in newborns compared with no immunization, according to a study published in JAMA.

Researchers conducted a prospective cohort study of 626 pregnancies to determine whether pertussis toxin antibody concentrations were present in cord blood from neonates born to mothers both immunized and unimmunized with Tdap vaccine. Women included in the study who delivered at ≥37 weeks’ gestation either received Tdap immunization between weeks 27 and 36 of gestation (Tdap-exposed group; n=312) and ≥14 days before delivery or did not receive Tdap vaccine during pregnancy (Tdap unexposed group; n=314). In addition, residual umbilical cord serum for serologic testing had to be available. Cord serum samples were collected from infants born from December 9, 2012, to March 15, 2014.

The primary outcome was pertussis toxin antibody concentrations found in cord blood from infants of Tdap-immunized mothers vs Tdap-unimmunized mothers; secondary outcome was to determine optimal gestation to administer Tdap vaccine that would result in maximum neonate pertussis toxin antibody concentration. Antibody concentrations were measured at 15 IU/mL or higher, 30 IU/mL or higher, and 40 IU/mL or higher.

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Mean gestation of maternal Tdap vaccine administration was 31.2 weeks; approximately 80% of participants received Tdap vaccine between weeks 28 and 32. Geometric mean concentrations (GMCs) of pertussis toxin antibodies of infants born to Tdap-immunized women was 47.3 IU/mL; for infants born to Tdap-unimmunized women, concentrations were 12.9 IU/mL. Overall, more infants born to Tdap-immunized women compared with unimmunized women had higher concentration levels: ≥15 IU/mL, 86% vs 37% (difference, 49%); ≥30 IU/mL, 72% vs 17% (difference, 55%); and ≥40 IU/mL, 59% vs 12% (difference, 47%; P <.001 for all).

Concentrations of pertussis toxin antibodies were highest when Tdap vaccine was administered at week 30 (GMC, 57.3 IU/mL). Investigators observed that after 30 weeks of gestation, GMC decreased significantly with increasing gestation age at immunization.

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The estimated GMC of serum pertussis toxin antibody at 2 months of age was 11.8 IU/mL in infants born to Tdap-immunized mothers and 3.2 IU/mL for those born to Tdap-unimmunized mothers. Pertussis toxin antibodies at infant age of 2 months were the highest when Tdap vaccine was administered during week 30 (14.3 IU/mL), followed by weeks 29 and 28.

“This study demonstrated that, following US immunization recommendations and in accordance with current understanding of the kinetics of placental transfer, optimal time to administer Tdap vaccine to maximize pertussis toxin antibodies at birth may be early in the third trimester, with the window of 27 through 30 weeks of gestation yielding the highest cord blood levels,” the investigators concluded.


Healy CM, Rench MA, Swaim LS, et al. Association between third-trimester Tdap immunization and neonatal pertussis antibody concentration. JAMA. 2018;310(14):1464-1470.