Becker-Dreps S, Butler AM, McGrath LJ, Boggess KA, Weber DJ, Li D, Hudgens MG, Layton JB. Effectiveness of prenatal Tdap immunization in the prevention of infant pertussis in the US. Presented at the 34th ICPE International Conference on Pharmacoepidemiology & Therapeutic Risk Management; August 25, 2018. Prague, Czech Republic. [abstract] Adv Pharmacoepidemiol Drug Saf. 2018; 27(S2):169. doi: 10.1002/pds.4629.

BACKGROUND: it is recommended that all pregnant women in the UnitedStates receive tetanus‐diphtheria‐acellular pertussis (Tdap) immuniza-tion to prevent infant pertussis. US recommendations state thatimmunization during any time of pregnancy is acceptable, althoughbetween 27 and 36 weeks was preferable.

OBJECTIVES:This study's objective was to examine the clinical effec-tiveness of prenatal Tdap, and whether effectiveness varies by gesta-tional age at immunization.Methods:A nationwide cohort study of pregnant women with deliver-ies in 2010‐2014 and their infants was performed. Commercial insur-ance claims data were analyzed to identify Tdap receipt by thepregnant women, and hospitalizations and outpatient visits for pertus-sis in their infants until 18 months of age. Pertussis occurrence wascompared between infants of mothers who received prenatal Tdap(overall, and stratified by gestational age at administration) and infantsof unvaccinated mothers. The duration of protection was evaluated bystratifying the infant's follow‐up 0‐2, 0‐6, and 6‐18 months.

There were 675 167 mother‐infant pairs in the cohort.Among infants whose mothers received prenatal Tdap, the rate of per-tussis was 42% lower (hazard ratio [HR] = 0.58, 95% CI: 0.38, 0.89)than infants whose mothers did not receive prenatal or postpartumTdap; this reduction was consistent across pertussis definitions (HRfor inpatient‐only pertussis = 0.50, 95% CI: 0.23, 1.09). Pertussis rateswere also lower for infants whose mothers received Tdap during thethird trimester. Infants whose mothers received Tdap <27 weeks ofgestation did not experience reductions in pertussis rates (HR for per-tussis = 1.06, 95% CI: 0.53, 2.15). Protective associations wereobserved when considering only 0‐2 and 0‐6 months of follow‐up inthe infants. No association, positive or negative, was observedbetween 6 and 18 months of follow‐up.

CONCLUSION: Infants of mothers who received prenatal Tdap experi-enced half the rate of pertussis as compared with infants of unimmu-nized mothers, and this reduction is most pronounced in the first fewmonths of life, typically before the infant's recommended receipt ofthe infant pertussis vaccination series. These results do not provideevidence to support changing the currently recommended timing ofTdap administration in pregnancy.

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