We estimated the effect of pertussis vaccination on reducing transmission from vaccinated breakthrough cases from a comprehensive follow-up of a community of 30,000 residents in Niakhar, Senegal. Using a wide spectrum of case definitions, vaccine efficacy was estimated as 1 - the ratio of secondary attack rates (SAR) in all households with cases during the calendar year 1993, a pertussis epidemic year. Vaccine efficacy for infectiousness (VEi) was 85% (95% confidence interval (CI), 46-95%) for children vaccinated with three doses of a whole-cell (WC; 94%) or an acellullar (6%) pertussis vaccine, with pertussis defined as a cough >/=21 days with paroxysms confirmed by culture, serology, or contact with a culture-confirmed person. It was high for all case definitions. Partial vaccination reduced infectiousness. Pertussis vaccination is highly effective in reducing transmission from vaccinated breakthrough cases.