Long-term clinical effectiveness of an acellular pertussis component vaccine and a whole cell pertussis component vaccine

The objective of this open study was to monitor the long-term effectiveness of the Lederle-Takeda diphtheria and tetanus toxoids and acellular pertussis antigen(s) (DTaP) vaccine and the Wyeth-Lederle diphtheria and tetanus toxoids and pertussis whole cell (DTP) vaccine in children who had received four doses of vaccine at 3, 4.5, 6 and 15 months of age during a pertussis vaccine efficacy trial from May 1991 to December 1994. After unblinding of the study code, follow-up information was obtained by use of standardised questionnaires twice a year from 1995 to 2000 to detect clinical pertussis and cough illnesses < or = 14 days duration. Physician confirmation was sought for all reported cases. Rates of reported cough illnesses < or = 14 days duration and rates of parent and physician diagnosed pertussis in former DTaP, DTP and diphtheria and tetanus toxoids (DT) recipients were determined and vaccine efficacy was calculated. Nine questionnaires were sent to parents of 2924 study children of whom 349, 1304 and 1271 had originally received DT, DTaP and DTP, respectively. Overall, rates for cough illnesses (per 100 person years) were similar among the vaccine groups suggesting that reporting bias was not a major factor. Calculated efficacy for the 6-year follow-up period based upon physician diagnosed pertussis was 89% (95% CI=79 94) for DTaP and was 92% (95% CI=84-96) for DTP. CONCLUSION: no evidence of decreasing efficacy over time was noted.