To investigate the possible dependence on age of the rate of decline of IgG antibodies to pertussis toxin (IgG-PT) after natural infection with Bordetella pertussis we measured IgG-PT in follow-up sera of 121 patients (age 0–94 years) obtained after 123 episodes of B. pertussis infection. For analysis we applied a dynamic model for the inactivation of B. pertussis by the immune system. There were no significant differences in rise, peak and decline of IgG-PT between different age groups, although there was a tendency for a more rapid increase, a higher peak and a faster decline with increasing age. The IgG-PT cut-off of 100 U/ml for serodiagnosis of pertussis appeared valid in all age groups. A decline of IgG-PT to <10 U/ml was associated with increased risk of re-infection with B. pertussis.
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