Diphtheria and tetanus toxoids and pertussis vaccine adsorbed (DTP): response to varying immunizing dosage and schedule.

In developing guidelines for the optimal schedule of DTP vaccine administration, it is imperative to explore alternative regimens to the current U.S. primary series of DTP vaccine at two, four, and six months and a booster at 18 months of age. Two approaches to primary immunization that reduce the total number of pertussis mouse protective units administered were studied in an effort to reduce adverse reactions. A modified, reduced (0.25 ml) dosage of each inoculation of DTP vaccine was compared with the usual immunization schedule. The reduced dosage group had consistently fewer adverse reactions with a significant reduction in febrile reactions and acute behavioural changes during the primary series at two, four, and six months and the booster at 18 months and fewer local reactions noted during the primary series (p less than 0.05). Geometric mean titers of pertussis agglutinins were higher after the primary series in vaccine recipients immunized with the recommended schedule but were similar in the two study groups before and after the 18 month immunization. An alternative approach studied the response following the administration of only two DTP immunizations of 0.5 ml, using a number of schedules to determine if lengthening the interval between the first and second pertussis inoculation would enhance the response to the pertussis antigen. Irrespective of whether the DTP vaccine was the first, second, or third of the primary series, the adverse reactions were similar. After the second DTP immunization, little difference was noted in the serologic response to pertussis. All agglutinin titers were significantly lower than that achieved after three standard doses of DTP vaccine (p less than 0.01).