School-age children and adolescents suspected of having been to be infected with pertussis in Japan.

Many countries including Japan have adapted acellular pertussis vaccines combined with diphtheria and tetanus toxoids (DTaP). DTaP vaccine coverage is approximately >90%, but pertussis re-emergence has been observed since 2000 in Japan. In the present study, anti-pertussis antibodies were investigated among school-age children and adolescents from 2013 to 2015. The positive rate of anti-pertussis toxin (PT) antibodies was higher among children aged 12-13 years (60.0%. 95%CI; 56.0-63.9%) in 2014 and 18-19 years (73.0%. 95%CI; 61.4-82.6%) in 2013, compared with 6-7 years (47.1%. 95%CI; 40.7-53.6%). The mean PT antibody titer was higher among children aged 12-13 years (23.8 EU/ml. 95%CI; 21.9-25.8) in 2014 and 18-19 years (29.3 EU/ml. 95%CI; 23.0-35.6) in 2013, compared with 6-7 years (18.3 EU/ml. 95%CI; 15.5-21.2). Distributions of pertussis antibodies and mean titers at their same grade of school-age were similar from 2013 to 2015. Although school-age children were immunized with 4 doses of DTaP, the data suggested the decay of vaccine-acquired immunity and possibility of asymptomatic infection in school age, indicating the additional DTaP vaccination before the entry of elementary school, preventing household contact.