AGE-SPECIFIC EFFICACY OF PERTUSSIS-VACCINE DURING EPIDEMIC AND NONEPIDEMIC PERIODS

A national survey was conducted of 3150 notified cases of whooping cou gh in order to determine age-specific pertussis vaccine efficacy by th e 'screening' method. The cases were collected over two periods, one j ust 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 qu estionnaire to the reporting doctor and clinical data were also collec ted to provide efficacy estimates according to standardized case defin itions. Overall, observed vaccine efficacy was high but differed betwe en epidemic (87 %) and non-epidemic (93 %) periods (P = 0.03). Efficac y 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 observe d from similar studies of children immunized at an accelerated schedul e and from phase III studies of acellular pertussis vaccines performed elsewhere.