Vaccine efficacy is 80%–85% following 3 doses of DTaP vaccine. Efficacy data following just 1 or 2 doses are lacking but are likely lower. The most effective way to prevent pertussis in early infancy is to vaccinate the mother between 27 and 36 weeks’ gestation. Antipertussis antibodies generated by the mother’s immune system are passed across the placenta to the fetus. One dose of Tdap should be administered during each pregnancy, preferably between 27 and 36 weeks’ gestation. Available data suggest that vaccinating closer to 27 weeks will maximize passive antibody transfer to the infant. A CDC evaluation found Tdap vaccination during the third trimester of pregnancy prevents 78% of pertussis cases in infants younger than 2 months of age and that maternal vaccination reduces the risk of infant hospitalization for pertussis by 90% (www.cdc.gov/pertussis/pregnant/hcp/vaccine-effectiveness.html).