• Sylvia Becker-Dreps, Anne M Butler, Leah J McGrath, Kim A Boggess, David J Weber, Dongmei Li, Michael G Hudgens, and J Bradley Layton.
• Department of Family Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina; Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina. Electronic address: sbd@unc.edu.
• Am J Prev Med. 2018 Aug 1; 55 (2): 159-166.

IntroductionIt is recommended that all pregnant women in the U.S. receive tetanus, diphtheria, acellular pertussis (Tdap) immunization to prevent infant pertussis. This study's objective was to examine the clinical effectiveness of prenatal Tdap, and whether effectiveness varies by gestational age at immunization.MethodsA nationwide cohort study of pregnant women with deliveries in 2010-2014 and their infants was performed. Commercial insurance claims data were analyzed in 2016-2017 to identify Tdap receipt by the pregnant women, and hospitalizations and outpatient visits for pertussis in their infants until the infants reached 18 months of age. Pertussis occurrence was compared between infants of mothers who received prenatal Tdap (overall and stratified by gestational age at administration) and infants of unvaccinated mothers.ResultsThere were 675,167 mother-infant pairs in the cohort. Among infants whose mothers received prenatal Tdap, the rate of pertussis was 43% lower (hazard ratio=0.57, 95% CI=0.35, 0.92) than infants whose mothers did not receive prenatal or postpartum Tdap; this reduction was consistent across pertussis definitions (hazard ratio for inpatient-only pertussis=0.32, 95% CI=0.11, 0.91). Pertussis rates were also lower for infants whose mothers received Tdap during the third trimester. Infants whose mothers received Tdap at <27 weeks of gestation did not experience reductions in pertussis rates (hazard ratio for pertussis=1.10, 95% CI=0.54, 2.25).ConclusionsInfants of mothers who received prenatal Tdap experienced half the rate of pertussis as compared with infants of unimmunized mothers. These results do not provide evidence to support changing the currently recommended timing of Tdap administration in pregnancy.Copyright © 2018 American Journal of Preventive Medicine. Published by Elsevier Inc. All rights reserved.

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