INFLUENCE OF HIGH TITERS OF MATERNAL ANTIBODY ON THE SEROLOGIC RESPONSE OF INFANTS TO DIPHTHERIA VACCINATION AT 3, 5 AND 12 MONTHS OF AGE

Diphtheria antitoxin was determined in serum from 44 pregnant women, o f whom 26 had received one injection of diphtheria toroid during pregn ancy, Their infants were vaccinated with a combined diphtheria-tetanus vaccine at 3, 5 and 12 months of age. This vaccination schedule has b een used in Sweden since 1986, replacing the old schedule of vaccinati on at 3, 4.5 and 6 months of age originally designed for diphtheria-te tanus-pertussis vaccine, which had not been used after cessation of ge neral vaccination against pertussis in 1979. Serum samples from the in fants were obtained at 3, 7 and 18 months of age. After 2 injections i nfants of mothers with high antitoxin titers, greater than or equal to 0.1 IU/ml, tended to have lower antitoxin titers than infants of moth ers with low antitoxin concentrations (P = 0.067). AU children had, ho wever, antitoxin above the minimum protective level of 0.01 IU/ml, Med ian antitoxin titers were 1.6 IU/ml in both groups after the third boo ster injection, Four infants of mothers who had been vaccinated during pregnancy and who had titers of greater than or equal to 0.4 IU/ml di d not reach the 0.1 IU/ml level after 2 injections: all 4 responded wi th high antitoxin titers after the third dose. Thus all infants were p rimed by 2 doses of vaccine, irrespective of maternal antibody concent ration, The repressive effect of maternal antibody on titers noted aft er 2 doses was no longer observed after the third, booster dose.