Long term serologic follow-up after pertussis immunization.

Neutralizing antibodies to pertussis toxin (antitoxin) were determined in 201 blood samples from 4-year-old children. They had received primary immunization at 6 to 8 months of age with an acellular (n = 149) or a whole cell (n = 52) pertussis vaccine and 195 of them had received a booster dose of the acellular vaccine 9 to 16 months later. Data on exposure to pertussis and occurrence of pertussis were also collected. There was a rapid decrease of antitoxin between immediate postbooster titers and those measured 24 months later. This decrease per month was significantly greater than that after the primary immunization series (P less than 0.001). Neither the number nor the spacing of acellular vaccine doses given for primary series influenced the titers found 24 months after the booster. An antitoxin response was still measurable in 86% of the 196 four-year-old children. None of 19 exposed children developed whooping cough, which suggested that the antibody concentrations during the follow-up period were sufficient for protection. The results indicate a need for long term follow-up studies in the evaluation of new vaccines and immunization schedules.