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In a previously reported double blind efficacy trial of a pertussis toxoid vaccine, 3450 infants were randomized to receive diphtheria-tetanus toxoids with or without pertussis toxoid at 3, 5 and 12 months of age. Efficacy against pertussis as defined by the World Health Organization was 71% from 30 days after the third vaccination with an average follow-up of 17.5 months. We now report efficacy for an additional 6 months of open follow-up.Parents were contacted monthly by a nurse. If a participant or a family member coughed for ≥7 days, a nasopharyngeal sample and paired sera were obtained.Efficacy during this open follow-up period was 77% (95% confidence intervals, 66 to 85%) based on 29 and 110 cases fulfilling the WHO definition of pertussis in vaccinated and control children, respectively. Efficacy against household exposure was 76% (95% confidence intervals, 51 to 91%). Pertussis in vaccinated children had a significantly shorter duration than pertussis in control children. Determination of pertussis toxin antibodies in paired sera with enzymelinked immunosorbent assay had a lower diagnostic sensitivity in vaccinated (45%) than in control (92%) children, while determination of antibodies against filamentous hemagglutinin (not included in the vaccine) was highly sensitive for diagnosing pertussis in both groups (100 and 90%, respectively).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 antigen that is not included in the vaccine.