Acellular vaccines against pertussis could be developed because various virulence factors of B. pertussis have been characterized. Acellular pertussis vaccines should retain the efficacy but have lower side effects, as compared to the conventional whole-cell vaccine. Lacking any correlate of antibacterial resistance, the efficacy of the vaccines had to be tested in large field trials. Such trials have been conducted and are being conducted in various European and in one African country. These trials used different designs, and various different vaccines were tested. All available efficacy data show that acellular pertussis vaccine can effectively protect against typical pertussis. It also seems probable that the efficacy of vaccines, which contain more than two pertussis components may be better than a vaccine containing pertussis toxoid or pertussis toxoid with filamentous hemagglutinin. A three-component acellular pertussis vaccine has been licensed for use in primary vaccination in infants in Germany in early 1995.