DTP reactions and serologic response with a reduced dose schedule.

In a double-blind study, infants received standard (0.5 ml) or modified (0.25 ml) doses of DTP vaccine for the primary series of three immunizations administered at 2, 4, and 6 months of age and the booster immunization at 18 months. Side effects and antibody responses were determined in 80 children who completed the primary series and 73 who received the booster. The modified regimen was associated with significantly reduced febrile reactions and behavioral changes after the primary series and booster inoculation: 63.2% of those who received the standard dose had febrile reactions, compared to 42.3% who received the modified dose during the primary series; a similar difference was observed with the booster. Only 47.2% of the reduced dosage recipients demonstrated marked behavioral changes, and 62.4% of the standard vaccine recipients had comparable reactions. An even larger difference (33.3% vs 64.7%) was noted at the time of the booster. The modified vaccine produced a local reaction incidence of 58.5%, compared to 72.6% in the control population during the primary immunization series; no differences were noted in local reactions with the booster dose. All patients had serologic evidence of protective titers against diphtheria and tetanus. After the primary immunization series, 97.6% and 97.3% of the infants given the modified and standard doses, respectively, had pertussis agglutinin titers of greater than or equal to 1:16. One patient who received the standard dosage had a titer of less than 1:16 one month after the booster immunization, whereas all those given the modified dose had titers greater than or equal to 1:16. Geometric mean titers of pertussis agglutinins were higher in the standard vaccine recipients after the primary series, but were similar in the two study groups before and after the 18-month immunization.