Studies on Diphtheria-Pertussis-Tetanus Combined Immunization in Children

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Abstract
Summary and Conclusions: The immune responses of 239 free-living children who received primary vaccination with 3 monthly doses of alum-precipitated DPT vaccine were analyzed on the basis of latent diphtheria immunity prior to vaccination. It was found that the latently diphtheria-immune group yielded significantly weaker pertussis agglutinin and tetanus antitoxin responses than the diphtheria non-immune group. The responses of the latently diphtheria-immune group showed the following characteristics: The magnitude of pertussis agglutinin formation was suppressed, with a mean titer ratio of 1:3.2 against the diphtheria non-immune group 1 mo after the third dose. During the period of observation, the interfered pertussis agglutinin response curve showed no tendency to cross the normal pertussis agglutinin response curve.Both speed and magnitude of tetanus antitoxin formation were heterologously interfered with, the most striking interference following the second dose (mean titer ratio 1:11.3). The interfered tetanus antitoxin response curve tends to cross the normal tetanus antitoxin response curve at their peaks.Weak but detectable interference of pertussis agglutinin and tetanus antitoxin responses occurs when there pre-exists the slightest degree of latent diphtheria immunity. As the degree of pre-existing diphtheria immunity increases, the interference intensifies. In our studies, a diphtheria antitoxin level of 0.01 u/ml represents that degree of immunity over and under which the intensity of interference differs significantly. A further graded increment of pre-existing diphtheria immunity above this level, however, does not result in proportional enhancement of interference. The significance of these observations is discussed.

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