Pertussis Maternal Immunization Study

Inclusion Criteria: * Pregnant women 18 years of age and over. * Women who, at ≥30-\<32 weeks gestation, are at low risk for complications as determined by the obstetrical algorithm for identification of eligible subjects and the obstetrical risk assessment form. * Signed, informed consent. Exclusion Criteria: * Failure to meet eligibility criteria as determined by the obstetrical algorithm for identification of eligible subjects and the obstetrical risk assessment form. * History of significant medical disorder (such as bleeding disorders, cancer, autoimmune disease, immunodeficiency (including HIV-infected individuals, transplant recipients), seizure disorder or significant psychiatric illness, drug or alcohol dependence). * Receipt of any high-dose daily corticosteroids (inhaled steroids are acceptable) within 2 weeks of study entry. High dose is defined as a dose of ≥20 mg of prednisone daily or equivalent. * History of physician-diagnosed or laboratory-confirmed pertussis within the past 5 years. * Personal history (verbal or documented) of ever having received Tdap. * Personal history (verbal or documented) of having received Td immunization within the past 2 years. * History of febrile illness (\>37.8ºC orally) within the past 72 hours (immunization may be deferred). * History of sensitivity to any component of Tdap. * Receipt of blood products or immunoglobulin within 3 months of study entry (except RH-negative women who receive immunoglobulin during pregnancy are eligible). * Receipt of any vaccines within 2 weeks of study vaccine (except influenza vaccine which may be given concurrently). * Failure to give written, informed consent.