Pertussis among the youngest infants in California: Evaluating the effectiveness of prenatal Tdap vaccination and identifying infants at greatest risk of disease by

Background: Most severe and fatal cases of pertussis occur in infants younger than eight weeks of age, before initiation of the primary pertussis vaccine series. To help protect infants from pertussis during this vulnerable period, women are recommended to receive Tdap vaccine at the start of the third trimester of each pregnancy to optimize transplacental transfer of antibodies to the fetus. This recommendation was made by the Advisory Committee on Immunization Practices based on immunogenicity data, and no studies in the United States have yet evaluated the effectiveness of this strategy in reducing pertussis incidence in infants. Methods: I evaluated a cohort of mothers with documented Tdap vaccination histories in the California Immunization Registry (CAIR) to determine if infants whose mothers received Tdap vaccine between 27 and 36 weeks gestation had a lower risk of pertussis in the first eight weeks of life, compared to infants born to women who received Tdap vaccine postpartum. Results: Prenatal Tdap vaccination during 27-36 weeks gestation was found to be 85% more effective at preventing pertussis in infants younger than eight weeks of age compared to Tdap vaccination postpartum. Vaccination received during 27-36 weeks gestation was more effective at preventing pertussis in infants than vaccination received prior to 27 weeks or after 36 weeks gestation. Conclusions: Tdap vaccination during 27-36 weeks gestation was 85% more effective at preventing pertussis in infants younger than eight weeks of age compared to Tdap vaccination postpartum. Efforts should be made by prenatal care providers to provide Tdap vaccine to pregnant women during routine prenatal visits at the earliest opportunity between 27-36 weeks gestation.