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Background. Antibiotic postexposure prophylaxis (PEP) following pertussis exposure is recommended but has never been evaluated in healthcare personnel (HCP) vaccinated with acellular pertussis vaccine (Tdap).Methods. Tdap-vaccinated HCP were randomized to receive azithromycin PEP or no PEP following pertussis exposure. Acute and convalescent nasopharyngeal swabs and sera were obtained for pertussis testing by polymerase chain reaction (PCR) and anti–pertussis toxin (PT) immunoglobulin G, respectively. A nasopharyngeal aspirate was also collected for PCR and culture from subjects who reported respiratory symptoms within 21 days following identification of the exposure. Pertussis infection was defined as a positive culture or PCR, a 2-fold rise in anti-PT titer, or a single anti-PT titer of ≥94 enzyme-linked immunosorbent assay units/mL. Daily symptom monitoring without PEP was considered noninferior to PEP after pertussis exposure if the lower limit of the 1-sided 95% confidence interval (CI) for the reduction in pertussis was greater than −7%.Results. During 30 months of study, 86 subjects were randomized following a pertussis exposure. Using the predefined definition of infection, pertussis infection did not develop in 41 (97.6%) of 42 subjects who received azithromycin PEP and 38 (86.4%) of 44 subjects who did not receive PEP (absolute risk difference, −11.3%; lower bound of the 1-sided 95% CI, −20.6%; P = .81). However, no subject developed symptomatic pertussis confirmed with culture or a specific PCR assay, and possibly no subject developed subclinical pertussis infection based upon additional serologic testing.Conclusions. Using the predefined definition of pertussis infection, noninferiority for preventing pertussis following exposure was not demonstrated for daily symptom monitoring of Tdap-vaccinated HCP without PEP when compared with antibiotic PEP. However, the small number of exposed HCP warrants further study of this approach.Clinical Trial Registration. NCT00469274.