Immunization of pregnant women with the Tdap vaccine during weeks 27-30 of pregnancy yielded the higher concentration of protective pertussis toxin antibodies in neonates, compared with neonates not exposed to the vaccine, based on data from more than 600 pregnancies.
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The Centers for Disease Control and Prevention recommends Tdap vaccination for pregnant women between 27 and 35 weeks’ gestation, but “data on pertussis antibody concentrations following maternal Tdap immunization are limited because of small cohort size or differing immunization schedules,” wrote C. Mary Healy, MD, of Baylor College of Medicine, Houston and her colleagues.
In an observational study published in JAMA, the researchers reviewed data from 626 pregnancies and births at a single tertiary care center in Texas. The average age of the mothers was 30 years; 41% were white, 27% Hispanic, 26% black, 5% Asian, and 1% another ethnicity. Of these, 312 received the Tdap vaccine at an average gestation of 31 weeks, while 314 women were not immunized.
Overall, the geometric mean concentration (GMC) of neonatal umbilical cord pertussis toxin antibodies among infants exposed to Tdap was 47.3 IU/mL, compared with 12.9 IU/mL for unexposed infants, yielding a GMC ratio of 3.6 (P less than .001).
In addition, pertussis toxin antibody concentrations of 15 IU/mL or higher, 30 IU/mL or higher, and 40 IU/mL or higher were significantly more common in Tdap-exposed vs. Tdap-unexposed neonates (86% vs. 37%, 72% vs. 17%, 59% vs. 12%, respectively).
The GMC of pertussis toxin antibodies was highest when the vaccine was given during weeks 27-30, after which the GMC declined, the researchers noted.
The study was limited by several factors including the observational design, which does not support causes and effect analysis, the use of data from a single center, and the lack of pre- and postimmunization serum samples for comparison, and the absence of data on women immunized during the second trimester, the researchers wrote.
However, the results support data from previous studies, and were strengthened by the large study population. The findings suggest that “following U.S. 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 researchers said.
The study was supported in part by the Centers for Disease Control and Prevention. Dr. Healy disclosed relationships with Sanofi Pasteur, Novartis Vaccines, Pfizer, and Novavax.
SOURCE: Healy C et al. JAMA. 2018 Oct 9;320(14):1464-70.