A cross sectional survey measuring sero-incidence of pertussis infection among Japanese junior and senior high school students in 2013 and 2014.

Pertussis in adolescents has been increasingly documented in recent years, but diagnosis from the clinical symptoms is difficult. Serological diagnosis with IgG antibody to pertussis toxin (IgG PT) is useful for detecting pertussis cases in this population. However, no serological criterion for recent infection has been fully validated and large-scale, longitudinal serological data among Japanese junior and senior high school students are lacking. Paired serum samples of 3243 junior and senior high school students, collected in 2013 and 2014, were analyzed for IgG PT and its relationship to possible risk factors. Regression analysis showed an average decrease of 35% in IgG PT between 2013 and 2014. In 2013, 4.4% of the students showed IgG PT levels ≥100EU/mL, as did 3.7% in 2014. The seroincidence, defined as [IgG PT] change from <100 in 2013 to ≥100EU/mL in 2014, was 10.3 cases per 1000 person-years. A 4-fold rise in IgG PT was seen in 2.1% of the students, with significant differences between schools and significant correlations to two risk factors, "over 2weeks coughing" and "exposure to a person with over 2weeks coughing". A substantial number of students had IgG PT ≥100EU/mL despite the observed 35% yearly decrease in IgG PT level. The local foci of ≥4-fold IgG PT increase in specific schools suggests the persistent circulation of B. pertussis in Japanese adolescents. The results also support a "≥4-fold rise in IgG PT" as a useful component of the sero-epidemiological surveillance for pertussis.