Pertussis antitoxin decay after vaccination with DTPa: Response to a first booster dose 312–612 years after the third vaccine dose

Longitudinal serum samples were collected from 542 children that had participated in a Swedish pertussis vaccine trial 1992–1995 [Gustafsson L, Hallander HO, Olin P, Reizenstein E, Storsaeter J. A controlled trial of a two-component acellular, a five-component acellular, and a whole-cell pertussis vaccine. N Engl J Med 1996;334(6):349–355] and who did not contract pertussis. The sera were analyzed for post vaccination antibody decay and for booster response of anti-PT (IgG antibodies against pertussis toxin), as measured by ELISA.Generally, an initial rapid decay of antitoxin antibody concentration was followed by a slower decay; the change occurring when the geometric mean level of antitoxin concentration reached 8–9 ELISA Units/mL (EU/mL). The time needed to reach this level was 8–9 months after the third dose in a 2, 4, and 6 months schedule.A “best-fit” combined regression model was used to predict when 50% of the children have less than the minimum level of detection of anti-PT (1 EU/mL). This occurred about 65 months after dose 3 at an age of 6 years.The anti-PT response to a booster dose was evident but the post-booster geometric mean values decreased with number of years after the third dose and the response appeared later.The results indicate that a pre-school booster might be considered at 6 years of age or earlier.