The incidence of Bordetella pertussis infections estimated in the population from a combination of serological surveys.

Objectives: Bordetella pertussis circulates even in highly vaccinated populations. There is a considerable amount of infection in adults. For designing more effective vaccination schedules it is important to quantify the age-dependent relation between the number of notified cases and the number of infections.

Methods: We used a statistical relationship between the time since infection and the IgG antibody titers against pertussis toxin, derived from a longitudinal data set, to estimate time since infection for all individuals in a cross-sectional population-based study (1995-1996) based on their titers. Age-specific incidence of infection with B. pertussis was calculated and compared with the age-distribution of notified cases of pertussis in 1994-1996.

Results: Estimated incidence of infection was 6.6% per year for 3-79-year olds, annual incidence of notified cases 0.01%. Estimated age-specific incidence of infection was lowest for 3-4-year olds (3.3%) and increased gradually up to the age of 20-24 years (10.8%). The number of notified cases was highest for 3-9-year olds.

Conclusions: In the Dutch population B. pertussis infections occur more frequently and in elder age-categories then suggested by notifications. Mathematical modeling could explore what booster vaccination strategies are most effective in reducing severe disease among not (completely) vaccinated infants.