Clustered cases of Bordetella pertussis infection cause high levels of IgG antibodies against pertussis toxin in adolescents in Gaobeidian city, China.

SUMMARY In a large serosurvey performed in Gaobeidian city, China, 1032 children and adolescents aged 3-18 years were divided into three age groups: 3-6, 7-11, and 12-18 years. The geometric mean of IgG antibodies against pertussis toxin (anti-PT IgG) varied from 3.47 IU/ml (3-6 years) to 8.71 IU/ml (12-18 years) and the seroprevalence (≥ 40 IU/ml) varied from 4.48% (7-11 years) to 11.76% (12-18 years). Infections were clustered in three secondary schools (schools 1-3). However, excluding secondary schools 1-3, the IgG PT levels, seroprevalence and rates of recent infections (>100 IU/ml) in secondary schools 4-7 (where infections were not clustered) were very close to those in the 3-6 and 7-11 years age groups. This suggests that the high seroprevalence and anti-PT IgG levels in adolescents in Gaobeidian city, China were caused by the clustered cases in certain secondary schools. Our results also suggested that pertussis was still circulating in China, and the reported incidence might be underestimated.