Unchanged efficacy of a pertussis toxoid vaccine throughout the two years after the third vaccination of infants

Background.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