Prenatal Tdap most effective early in third trimester

Maternal immunization with Tdap during pregnancy can provide neonates with higher levels of pertussis toxin antibodies, which protect them from infection, according to research published in JAMA. Researchers said that infants received the highest concentrations of these antibodies when mothers were vaccinated early in the third trimester.

The CDC has recommended vaccinating pregnant women with Tdap between 27- and 36-weeks’ gestation since 2013, but the optimal gestation for immunization has been unclear, the researchers added.

“Third-trimester Tdap immunization was shown to be safe, allowed efficient placental transport of induced maternal antibodies and did not interfere significantly with infant antibody response to pertussis toxin during the primary immunization series,” C. Mary Healy, MD, assistant professor of infectious diseases at Baylor College of Medicine’s department of pediatrics, and colleagues said. “However, data on pertussis antibody concentrations following maternal Tdap immunization are limited because of small cohort size or differing immunization schedules.”

Healy and colleagues conducted a prospective, observational cohort study that included term infants born in Houston, Texas, between December 2013 and March 2014. The researchers used cord blood from infants exposed to and unexposed to Tdap and calculated the proportion of infants with pertussis toxin antibody concentrations reaching 15 IU/mL or higher, 30 IU/mL or higher or 40 IU/mL or higher.

Of the 626 pregnancies included in the study, 312 women received prenatal Tdap vaccination (mean gestation, 31.2 weeks). Infants who were prenatally exposed to Tdap had a geometric mean concentration (GMC) of 47.3 IU/mL. Those who were not exposed to the vaccine had a GMC of 12.9 IU/mL.

Compared with infants born to unvaccinated mothers, more infants who were exposed to Tdap reached antibody concentrations of 15 IU/mL or higher (86% vs. 37%), 30 IU/mL or higher (72% vs. 17%) and 40 IU/mL or higher (59% vs. 12%). Healy and colleagues observed that infants born to mothers who received Tdap vaccination between 27- and 30-weeks’ gestation had the highest GMC of pertussis toxin antibodies, peaking at week 30 (57.3 IU/mL). The GMC of pertussis toxin antibodies in cord sera declined after that, the researchers said.

“Pertussis toxin is the pertussis antigen most associated with severe infant disease. Although no definitive serologic correlate of immunity for pertussis has been established, it is likely that antibody concentrations needed to protect young infants are higher than those required for older children and adults,” the researchers wrote. “Pertussis toxin antibody concentrations at birth were sufficiently high in infants born to Tdap-immunized mothers that, even allowing for the natural decay of maternal antibodies, most infants would have had substantial antibody levels until initiation of the primary vaccine series, thus reducing their risk of pertussis-related mortality and morbidity.” – by Katherine Bortz

Disclosures: Healy reports receiving research grants from Sanofi Pasteur and Novartis Vaccines and serving on advisory boards for Novartis Vaccines, Novavax Inc. and Pfizer Inc. Please see the study for a list of all other authors’ relevant financial disclosures.