Each vaccine is normally rated according to its efficacy and to the frequency and severity of unwanted side effects. The efficacy of whole-cell pertussis vaccines is beyond doubt, its frequent side effects, however, stimulated research into less reactogenic acellular vaccines. Several acellular pertussis vaccines of different composition have been developed. All of these produced significantly less local and systemic side effects, as compared to the conventional whole-cell vaccine. Various epidemiological data strongly suggest the efficacy of these vaccines. An equivalence of protective efficacy between whole-cell vaccines and acellular pertussis vaccines can, however, only be documented in controlled field trials, which are now ongoing in several countries.