UNCHANGED EFFICACY OF A PERTUSSIS TOXOID VACCINE THROUGHOUT THE 2 YEARS AFTER THE 3RD VACCINATION OF INFANTS

Background. In a previously reported double blind efficacy trial of a pertussis toroid vaccine, 3450 infants were randomized to receive diph theria-tetanus toxoids with or without, pertussis toroid at 3, 5 and 1 2 months of age. Efficacy against pertussis as defined by the World He alth Organization was 71% from 30 days after the third vaccination wit h an average follow-up of 17.5 months, We now report efficacy for an a dditional 6 months of open follow-up. Methods. Parents were contacted monthly by a nurse, If a participant or a family member coughed for gr eater than or equal to 7 days, a nasopharyngeal sample and paired sera mere obtained. Results. Efficacy during this open follow-up period wa s 77% (95% confidence intervals, 66 to 85%) based on 29 and 110 cases fulfilling the WHO definition of pertussis in vaccinated and control c hildren, respectively. Efficacy against household exposure was 76% (95 % confidence intervals, 51 to 91%), Pertussis in vaccinated children h ad a significantly shorter duration than pertussis in control children . Determination of pertussis toxin antibodies in paired sera with enzy me-linked immunosorbent assay had a lower diagnostic sensitivity in va ccinated (45%) than in control (92%) children, while determination of antibodies against filamentous hemagglutinin (not included in the vacc ine) was highly sensitive for diagnosing pertussis in both groups (100 and 90%, respectively). Conclusions. A monocomponent pertussis toxoid vaccine induces significant protection against pertussis for at least 2 years after the third injection. To obtain an unbiased estimate of vaccine efficacy it is important to determine antibodies against an an tigen that is not included in the vaccine.