Pertussis, caused by the bacterium Bordetella pertussis, is an acute respiratory tract infection transmitted to susceptible persons through aerosolized respiratory droplets and direct contact with respiratory secretions. Classic pertussis disease is characterized by three phases of illness: the catarrhal phase where persons note cough and coryza; the paroxysmal phase where persons develop a spasmodic cough with post-tussive vomiting and an inspiratory whoop; and the convalescent phase, during which symptoms slowly resolve. The risk of transmission of the organism is compounded by the nondescript nature of symptoms early in the course of illness, particularly in adults. Classically recognized as a disease of infants and children ("whooping cough"), the incidence of pertussis infection in adolescents and adults has increased in recent years. Persons \>15 years of age now make up more than twenty percent of reported cases. This increase is likely due to several factors, including waning protection from childhood vaccination and natural infection, an increased appreciation for disease in adolescents and adults, and the improved ability of clinicians to diagnose pertussis recognition through the use of serologic methods.

Healthcare workers (HCWs) are at increased risk for acquiring pertussis infection due to regular contact with infected patients and waning protection from childhood vaccination or from natural pertussis infection. Healthcare-associated outbreaks of pertussis have also been increasingly recognized and have been reported from a diverse range of healthcare facilities. Such outbreaks are often due to under-recognition of pertussis with subsequent failure to isolate suspected cases, waning immunity from childhood vaccination or disease, and the increasing incidence of pertussis infection in adults and adolescents. Infected HCWs can then serve as vectors of infection to other susceptible contacts including patients, other employees, and even their own children.

Vaccination is an effective tool for the prevention of pertussis. In 2005, two tetanus toxoid, reduced diphtheria toxoid, and reduced antigen quantity acellular pertussis vaccines (Tdap) were licensed for use in adolescents and adults. In view of the increasingly recognized problem of healthcare-associated and transmitted pertussis infection, the Centers for Disease Control and Prevention's (CDC) Advisory Committee on Immunization Practices (ACIP) targeted HCWs as a priority group for pertussis vaccination in 2006, primarily to reduce the risk of spread of pertussis within health care institutions.

Until the licensure of Tdap, the only method to reduce transmission of pertussis after healthcare-associated exposure to persons with pertussis was post-exposure prophylaxis (PEP) with antibiotics and employee furlough. Close contacts exposed to a pertussis-infected patient or staff member are routinely treated with macrolide therapy (erythromycin or azithromycin), and exposed HCWs who develop a cough-illness are restricted from work for 5 days while on antibiotic therapy. PEP is believed to prevent symptomatic infection in the exposed person if administered within 21 days of cough onset. Traditionally, decisions regarding PEP for exposed HCWs involve detailed assessments of the degree of patient contact, the risk for development of severe or complicated pertussis, and regular evaluation and follow-up for the occurrence of symptoms. These are often time-consuming efforts that are usually the responsibility of infection control or occupational health personnel. With the licensure of Tdap and with the recommended vaccination of HCWs, it is now hoped that vaccination will eliminate the need to provide antibiotic PEP, particularly in recently-vaccinated HCWs. However, this has not been confirmed with a randomized clinical trial, and, therefore, no definitive formal recommendation can be made regarding modifications of PEP in vaccinated HCWs. Two potential strategies exist for the management of vaccinated HCWs following an exposure to a person with pertussis: a) provision of universal antibiotic therapy or b) careful daily observation of vaccinated HCWs for the development of symptoms without antibiotic prescription. A comparison of these two strategies will be the focus of this study.