Age-specific efficacy of pertussis vaccine during epidemic and non-epidemic periods.

A national survey was conducted of 3150 notified cases of whooping cough in order to determine age-specific pertussis vaccine efficacy by the ''screening'' method. The cases were collected over two periods, one just prior to the start and one at the first peak of the whooping cough epidemic of 1989-90. Vaccination status was determined by a postal questionnaire to the reporting doctor and clinical data were also collected to provide efficacy estimates according to standardized case definitions. Overall, observed vaccine efficacy was high but differed between epidemic (87%) and non-epidemic (93%) periods (P = 0.03). Efficacy estimates were generally higher for typical or severe cases than for children with an atypical illness. Vaccine efficacy declined with age (P < 0.01) but estimates remained high up to the age of 8 years. This study will provide baseline data for comparison with efficacy observed from similar studies of children immunized at an accelerated schedule and from phase III studies of acellular pertussis vaccines performed elsewhere.