INCIDENCE OF PERTUSSIS IN THE LOWER LEFT RHINE REGION

Background: Four independent approaches were used to estimate the inci dence of pertussis in the Krefeld-Viersen area, an industrialized regi on comprising more than 500 000 people on the western border of the Rh ine-Ruhr basin. Methods: Isolation rates of B. pertussis and B. parape rtussis were monitored over a four year period and related to the numb er of practising pediatricians in the area. School entrance examinatio n data, which were collected by local health authorities at Krefeld an d Viersen from all children before age six, were screened for parental information about pertussis and for vaccination history. Case histori es of all three and six year old patients, admitted for the first time to a central community hospital (n=610) were examined for parental in formation on whooping cough. Finally, 48 pediatricians in free practic e were asked for an estimate of cumulative pertussis incidence at age six in a telephone poll. Results: Based on the isolation of Bordetella e from nasopharyngeal swabs, and supposing a case/swab ratio of 1.0, t he bacteria were found at a yearly average of 2.6% in the population u nder six years of age. Assuming a sensitivity of 50% for the isolation of Bordetellae, the annual incidence rate would be 5.2%. School entra nce data revealed that approximately 35% of the 6.5 year old children had pertussis according to parental information, which corresponds to an annual incidence rate of 5.9%. Hospital case histories showed that 28% of the six-year old children and 15% of the three-year old childre n had a history of pertussis. First time hospitalization data would po int to an annual incidence rate of 4.8%. At the telephone poll, the pe diatricians estimated a median of 26% of children having had pertussis at age six, corresponding to an annual incidence rate of 4.4%. Conclu sion: Thus, all four approaches yielded an estimated average annual in cidence rate of 4-6% per year in the first years of life and suggested in this consistency that this approximates the true crude incidence r ate in the area. As a gross estimate of vaccine efficacy, whole-cell p ertussis vaccine was able to prevent 75-80% of clinical disease accord ing to parental recall.