Diphtheria antitoxin response to DTP vaccines used in Swedish pertussis vaccine trials, persistence and projection for timing of booster

Data from two Swedish pertussis vaccine trials with various combination vaccines were used to compare anti-diphtheria antitoxin concentrations over time between different vaccines, vaccine lots and vaccine schedules. The immune responses were measured with a validated ELISA method.Results are given for 1326 children, born 1992, that were recruited to the placebo (DT)-controlled Trial I which used a 2, 4, 6 month schedule. Two DTP acellular and one DTP whole cell vaccine were used. No DT boosters were given until 5 years of age.Trial II recruited children born 1993-94 and compared three DTP acellular vaccines with one DTP whole cell vaccine. Results are given for 306 children in a 2, 4, 6 month schedule and for 531 children in a 3, 5, 12 month schedule. The latter schedule gave significantly higher diphtheria antitoxin concentrations post third dose.The various DTP acellular vaccines and an inefficacious DTP whole cell vaccine gave lower antitoxin concentrations than both an efficacious DTP whole cell vaccine and the DT vaccine. The larger differences in antigen response between vaccines was reduced in the course of time. Generally, an initial rapid decline of antitoxin concentration was followed by a slower decline; the change typically occurring when the antitoxin concentration reached 0.13-0.16 EU/ml. The time needed to reach this level was between 6 and 10 months based on the initial vaccine response.A "best-fit" combined exponential regression model was used to predict the optimal timing for booster vaccinations against diphtheria.Our data support a 3, 5, 12 month schedule followed by a fourth dose 4-5 years after the third dose, depending upon the vaccine used.