Efficacy of whole-cell pertussis vaccine in preschool children in the United States.

OBJECTIVE To evaluate the efficacy of currently used whole-cell pertussis vaccines. DESIGN Active surveillance to detect pertussis cases in Baltimore, Md, Denver, Colo, and Milwaukee, Wis, and investigation of secondary attack rates in 347 household contacts, aged 1 through 4 years, to estimate vaccine efficacy. OUTCOME MEASURE Vaccine efficacy was estimated using different case definitions for pertussis. RESULTS Vaccine efficacy was 64%, 81%, and 95% for case definitions of mild cough, paroxysmal cough, and severe clinical illness, respectively. Requiring laboratory confirmation increased efficacy to 95% to 98% for culture-positive children and to 77% to 95% for culture- or serology-confirmed cases, depending on disease severity. Vaccine efficacy for typical paroxysmal cough increased from 44% for one diphtheria, tetanus, and pertussis vaccine dose to 80% for four or more doses. CONCLUSIONS The trend toward increasing vaccine efficacy with different case definitions may be due to improved efficacy in preventing severe illness and to case definitions that are more specific for pertussis. Whole-cell pertussis vaccine was highly effective in preventing pertussis in preschool children exposed to infection within their households. Direct side-by-side efficacy studies of whole-cell vaccine and the recently licensed acellular vaccine will be necessary to assure that comparable protection is afforded by the new vaccines if they are to be used for immunization of infants.