(P21) Trends in Chlamydia trachomatis IgG seroprevalence in the general population of the Netherlands over 20 years

Background
Widespread Chlamydia trachomatis (Ct) screening in women and heterosexual men is under debate in several countries, due to lack of evidence for reduction in population prevalence. Ct reporting rates have been rising in the Netherlands, and it is poorly understood to what extent this reflects increased Ct transmission or improved case finding and reporting. Repeated nation-wide surveys are needed to put reporting data in perspective. We aim to provide insight in national Ct IgG seroprevalence (a marker of lifetime Ct exposure) during the period 1996-2017.

Methods
From three independent nation-wide population-based studies in 1996, 2007 and 2017, serum samples of 4300 men and women aged between 15-39 years were drawn. Participants completed a questionnaire on demographics and sexual behaviour. Serum antibodies were analysed using Medac Ct IgG ELISA test. Census weights were assigned to achieve seroprevalence estimates representative of the general Dutch population. Seroprevalence estimates were stratified by gender, age and birth cohort. Trends and risk factors in young men and women (< 25 years old) were explored using multivariable logistic regression.

Results
Among young women (15-24 years) seroprevalence was relatively low at 5.9% [3.7%-9.2%] in 1996, yet increased to 7.6% [5.1%-11.2%] in 2007 and 8.8% [5.5%-13.9%] in 2017. Among older women (25-39 years), the seroprevalence significantly decreased from 15.6% [12.2%-19.7%] in 1996 to 9.5% [7.2%-12.4%] in 2007 and remained lower in 2017 (11.2 [7.2%-12.4%]). Seroprevalence increased with age in the most recent female birth cohort (1977-1981; (p=0.01)). The overall seroprevalence was higher in females (10.5% [9.2%-12.0%]) than in males (5.8% [4.7%-7.0%]). For males the seroprevalence was similar across age groups and over the study period. Determinants of seropositivity in young men and women were: non-Western migration background (aOR: 2.4 [1.2-4.8]), ever prior STI (aOR: 4.7 [1.6-10.7]), and ≥2 recent sex partners (aOR: 2.2 [1.1-4.5]). No interaction with study period was observed.

Conclusion
Our results suggest that Ct seroprevalence was historically high, dropped until the early 2000s, but may be resurging in youngsters (< 25 years) in more recent years. The increase in seroprevalence suggests an increase in Ct transmission despite intensified testing in the Netherlands.