Evaluation of pertussis immunity status in schoolchildren immunized with whole-cell vaccine.

It has recently been reported that the worldwide increase in the number of pertussis cases is a result of the waning of whole-cell vaccine-induced immunity. Thus, in this study, we aimed to investigate the pertussis immunity status of primary and secondary school students in a district of Ankara, Turkey. A total of 997 healthy students, aged 9-17 years, who had been immunized with four doses of whole-cell pertussis vaccine were included in the study. The subjects were divided into two age groups: 9-14 and 15-17 years. To determine the immune status, serum levels of IgG anti-pertussis toxin (aPT) antibody were tested by in-house ELISA and arbitrarily evaluated as non-immune [< 10 ELISA units (EU)/ml], immune (10-100 EU/ml), and recent infection (> 100 EU/ml). Serum samples of 997 students (559 females, 438 males) aged between 9 and 17 years (mean 13.02 +/- 2.25, median 13 years) were tested. Non-immune, immune and recent infection levels of aPT were found in 27.3%, 59.3% and 13.4% of individuals, respectively. The immune group did not have statistically significant differences between males and females (P = 0.68). In the 9-14 and 15-17 years age groups, serum aPT antibody levels 10 EU/ml were 73.1% and 72.2%, respectively, which did not represent any statistical difference (P = 0.81). Students aged 15-17 years had a higher immunity rate than the 9-14 years group, and the percentage of students with recent infection in the 9-14 years group was higher than the 15-17 years group (P < 0.001). The peak age of non-immunized subjects was 9 years (47.0%), and decreased to a minimum at age 12-13 years, and began to increase again from age 13-14 years. In contrast, the ratio of recent infection was least at age 9-10 years, began to increase, and reached a peak at 12 years, and then decreased. On the other hand, it was observed that household size and monthly income were not associated with the immunity status (P = 0.65, P = 0.37, respectively). The results of the present study show that levels of antibody against pertussis decreased in the younger age groups and, as a result, there is an increase in the number of pertussis cases. Thus, in order to decrease the incidence of pertussis and protect infants, we recommend the application of booster doses at regular intervals.