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To investigate the duration of antitoxin response and immunity to pertussis 10 years after priming with either two or three doses of a two component acellular pertussis vaccine or with three doses of a whole cell vaccine and then boostered with the acellular vaccine.At 11 years of age 207 of 304 (68%) children of the original cohort returned for a blood sample and 262 (86%) participated by answering a questionnaire.Neutralizing antibodies to pertussis toxin (antitoxin) were analyzed by the Chinese hamster ovary cell assay. Clinical pertussis and pertussis exposure were investigated by a structured questionnaire.Measurable antitoxin was found in 77% of the samples, with no differences by type of vaccine or by the number of doses given for priming. A significant decrease of antibody concentration (P < 0.001) was noted from the previous recall at 4 years of age, and significant titer rises were found for 14% of the children, irrespective of known exposure. Confirmed pertussis during follow-up, as defined in the study, was reported for 14 of 262 (5%) children.The study showed that antitoxin concentrations are maintained in a situation of endemic pertussis and indicated that the long term protection after an acellular booster was good, irrespective of type of vaccine or the number of doses of acellular vaccine given for priming.