Late pregnancy pertussis vaccination may boost passive immunity

1. Women who received the dTpa vaccine in pregnancy had higher levels of cord antibodies to pertussis toxin (PT), pertactin (PRN), and filamentous hemagglutinin (FHA).

2. Cord antibodies were higher in women who received the dTpa vaccine early in the third trimester compared to those who received it late in the third trimester.

Evidence Rating Level: 1 (Excellent)

Study Rundown: While the advent of the pertussis vaccine greatly reduced mortality associated with pertussis, also known as whooping cough, there has been a gradual increase in reported cases of pertussis in recent years. Among those at greatest risk for pertussis-related morbidity and mortality are infants younger than 3 months. They have not yet actively developed sufficient levels of antibodies, are too young to receive the vaccination, and therefore depend on the herd immunity of caretakers as well as passive immunity acquired from their mothers to fight off infections. Maternal vaccination has been identified as an effective intervention to reduce the incidence of pertussis in the pediatric population. Current guidelines advise administration of the Tdap vaccine between 27 and 36 weeks to maximize antibody transfer to the newborn during the third trimester but the optimal timing of vaccination in the third trimester is unknown. In the present work, authors compared the efficacy of dTpa immunization during the early and late third trimester and found that earlier immunization was associated with higher antibody levels.

Strengths of the study were prospective design and inclusion of a non-intervention group for comparison. The study was limited by selection bias; it is unclear whether the characteristics of women who declined to participate differed from those who assented and whether these may influence cord antibody levels. Additionally, the control group was not matched to the intervention groups by demographic characteristics. Long-term follow-up to evaluate the incidence of pertussis in this population, as well as larger population studies, are needed to further inform recommendations regarding the optimal timing for immunization.

Click to read the study in AJOG

Relevant Reading: The effect of timing of maternal tetanus, diphtheria, and acellular pertussis (Tdap) immunization during pregnancy on newborn pertussis antibody levels – A prospective study

In-Depth [prospective cohort]: This study compared immunity to pertussis by cord antibody levels among women who did not receive the pertussis vaccine during pregnancy (n = 39), women who received the dTpa vaccine between 28 and 32+6 weeks gestation (n = 53), and women who received the same vaccine between 33 and 36+6 weeks gestation (n = 62). The outcomes of interest were cord antibody levels to PT, PRN and FHA at the time of delivery.

Women who received the dTpa vaccine during pregnancy had higher levels of all 3 antibodies as measured in cord blood at the time of delivery (p ≤ 0.001). Cord anti-PRN levels were higher among women who received dTpa between 28 and 32 weeks (p = 0.03). Antibody levels to PT trended towards being higher in the early vaccination group but was not statistically significant after adjustment for pre-vaccination antibody level.

Image: PD

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