COMBINED IMMUNIZATION AGAINST DIPHTHERIA, TETANUS AND PERTUSSIS IN NEWBORN INFANTS
Authors: DI SANT'AGNESE PAUL A. 
Published: December 2021

Additional serologic studies are presented of a group of newborn infants whose antibody production following combined prophylactic inoculation against tetanus, diphtheria and pertussis was reported in a previous paper.
Duration of Antibody Titers: In the 10 months following the last injection of triple combined antigen a steady decline in diphtheria antitoxin titers was observed which was more marked in patients who had achieved high antibody levels. A similar decrease was found in the percentage of infants with high titers of tetanus antitoxin, but there were no cases whose tetanus antitoxin level dropped to less than the "protective" titer (0.1 unit/cc.). Progressive decrease in diphtheria and tetanus antitoxin titers with passage of time is in agreement with findings of others.
After the third and last immunizing injection, a rapid initial decrease was noted in the number of patients with "protective" pertussis agglutinin titers (1:400 or higher); then a levelling off took place and no further change was noted in the next six months. On the other hand, a steady decline was found in the percentage of infants with "high" agglutinin levels (1:3200). To our knowledge this has not been observed before. The young age of our patients at the time of the basic injections may have been responsible for the findings.
Antibody Titers After Booster Dose: One group of infants was reinjected at the age of six months (four months after the third and last immunizing injection), another group at one year of age (10 months after the last injection). All booster doses consisted of 0.5 cc. of the same triple combined antigen used in basic immunization.
After booster a marked increase was noted in diphtheria antitoxin titers to a level higher than that observed following the basic immunizing injections. Tetanus antitoxin response was considered to have been equally good, although more difficult to evaluate because of the high antitoxin levels present before reinjection.
In the case of pertussis agglutinins, it appeared as if there were a "ceiling" of about 60% of infants who could, even after reinjection, develop a "protective" agglutinin titer (1:400 or higher).
A striking difference was observed in both pertussis agglutinin levels and diphtheria antitoxin titers achieved by infants reinjected at six months and one year of age. This was thought to be due to immaturity of the immune mechanisms in the younger age group. An added factor in the case of diphtheria antitoxin in some patients may have been the persistence of passive antibodies acquired transplacentally.
Antibody titers also were determined six months after booster dose in the infants who had been reinjected at the age of six months. A marked decrease was observed in the percentage of patients with "protective" pertussis agglutinin titers and "high" (1.0 unit/cc.) diphtheria antitoxin levels. No reduction was noted in tetanus antitoxin titers.
Effects of Passive immunity to Diphtheria on Active immunization with Diphtheria Toxoid: With only one exception, all infants tested after a booster dose had been administered between 6 and 12 months of age had "protective" diphtheria antitoxin titers (0.03 units/cc. or more). Active immunization against diphtheria was therefore considered to have been achieved in all cases (with one exception) despite the passively transmitted antitoxin present at birth in over half the cases.
While passive diphtheria antitoxin present at birth did not prevent "sensitization" of the antibody-forming tissues to the diphtheria antigen, it did decrease significantly the amounts of antitoxin actively produced in response to basic inoculation.
Reasons for the success of active diphtheria immunization in this series are discussed. Arguments against active immunization of mothers in pregnancy for protection of their offspring are considered.