The aim of this study was to assess the level of humoral immunity against diphtheria and pertussis by measuring IgG to diphtheria toxoid (DT) and pertussis toxin (PT) in general population of Beijing. A total of 2147 subjects aged 0-74 y were selected with a random sample of resident population in Beijing. The information of socio-demographic characteristics, vaccination history, disease history of diphtheria and pertussis were collected for each subject by questionnaire. Serum samples were tested for IgG antibodies to DT and PT by using commercial ELISA kits. The overall positivity rate of anti-DT IgG was 66.28% with the mean concentration of 2.169 IU/ml. Age stratified data showed that the highest positivity rate of 97.63% was observed in 1-4 y and the rates decreased with age. The positivity rates were only around 50% or below since 25 y old. The positivity rate of anti-PT IgG was 12.34% with the mean concentration of 15.163 IU/ml. The highest level of positivity rate (22.23%) and antibody level (23.101 IU/ml) was seen in <1 year old. In subjects older than 10 y old, the anti-PT IgG positivity rate was 10.19%-13.51% and concentration was 13.295 IU/ml -16.353 IU/ml, with no significant differences between these groups (χ2 = 1.664, P = 0.948; F = 0.369, P = 0.899). The subjects with anti-PT IgG ≥ 100 IU/ml were observed in nearly all the groups older than 5 y except for 10-14 age group. The estimated incidences of pertussis infection were higher than 6000/100000 in these age groups. A sharp increase of immunity level of diphtheria was observed at 1 y and 6 y respectively, which was consistent with the current immunization schedule. But there was no significant increase of immunity to pertussis observed after booster immunization at 18-24 months, but the proportions of undetectable were lowest in <1, 1, 2 years in children <14 years. As shown in the present study, the adult population was generally lack of protective antibody against diphtheria and all the age groups showed a low immunity to pertussis indicating the potential risk of transmission and outbreaks of the 2 diseases in Beijing.
Keywords: diphtheria; pertussis; seroepidemiology.